It is possible to downgrade. But definitely not recommended. You will rip big security holes into your system as also other packages needs downgrade. Better retire your 20 years old museum piece. Or run Leap 15.1 in a VM as already suggested.
Possible way to downgrade:
leaptest:~ # zypper ar -f https://download.opensuse.org/distribution/leap/15.1/repo/oss/ oss
Adding repository 'oss' ................................................................................................................................................................................................................................................[done]
Repository 'oss' successfully added
URI : https://download.opensuse.org/distribution/leap/15.1/repo/oss/
Enabled : Yes
GPG Check : Yes
Autorefresh : Yes
Priority : 99 (default priority)
Repository priorities are without effect. All enabled repositories share the same priority.
leaptest:~ # zypper ar -f https://download.opensuse.org/distribution/leap/15.1/repo/non-oss/ non-oss
Adding repository 'non-oss' ............................................................................................................................................................................................................................................[done]
Repository 'non-oss' successfully added
URI : https://download.opensuse.org/distribution/leap/15.1/repo/non-oss/
Enabled : Yes
GPG Check : Yes
Autorefresh : Yes
Priority : 99 (default priority)
Repository priorities are without effect. All enabled repositories share the same priority.
leaptest:~ # zypper in --oldpackage http://download.opensuse.org/distribution/leap/15.1/repo/oss/x86_64/sane-backends-1.0.27-lp151.5.3.x86_64.rpm
Refreshing service 'NVIDIA'.
Refreshing service 'openSUSE'.
Warning: The gpg key signing file 'repomd.xml' has expired.
Repository: non-oss
Key Fingerprint: 22C0 7BA5 3417 8CD0 2EFE 22AA B88B 2FD4 3DBD C284
Key Name: openSUSE Project Signing Key <opensuse@opensuse.org>
Key Algorithm: RSA 2048
Key Created: Mon May 5 10:37:40 2014
Key Expires: Thu May 2 10:37:40 2024 (EXPIRED)
Rpm Name: gpg-pubkey-3dbdc284-53674dd4
Retrieving repository 'non-oss' metadata ...............................................................................................................................................................................................................................[done]
Building repository 'non-oss' cache ....................................................................................................................................................................................................................................[done]
Warning: The gpg key signing file 'repomd.xml' has expired.
Repository: oss
Key Fingerprint: 22C0 7BA5 3417 8CD0 2EFE 22AA B88B 2FD4 3DBD C284
Key Name: openSUSE Project Signing Key <opensuse@opensuse.org>
Key Algorithm: RSA 2048
Key Created: Mon May 5 10:37:40 2014
Key Expires: Thu May 2 10:37:40 2024 (EXPIRED)
Rpm Name: gpg-pubkey-3dbdc284-53674dd4
Retrieving repository 'oss' metadata ...................................................................................................................................................................................................................................[done]
Building repository 'oss' cache ........................................................................................................................................................................................................................................[done]
Loading repository data...
Reading installed packages...
Resolving package dependencies...
Problem: 1: the to be installed sane-backends-1.0.27-lp151.5.3.x86_64 requires 'libnetsnmp.so.30()(64bit)', but this requirement cannot be provided
not installable providers: libsnmp30-5.7.3-lp151.7.5.x86_64[oss]
Solution 1: Following actions will be done:
downgrade of snmp-mibs-5.9.4-150600.24.2.1.x86_64 to snmp-mibs-5.7.3-lp151.7.5.x86_64
deinstallation of libsnmp40-5.9.4-150600.24.2.1.x86_64
downgrade of net-snmp-5.9.4-150600.24.2.1.x86_64 to net-snmp-5.7.3-lp151.7.5.x86_64
downgrade of perl-SNMP-5.9.4-150600.24.2.1.x86_64 to perl-SNMP-5.7.3-lp151.7.5.x86_64
Solution 2: do not install sane-backends-1.0.27-lp151.5.3.x86_64
Solution 3: break sane-backends-1.0.27-lp151.5.3.x86_64 by ignoring some of its dependencies
Choose from above solutions by number or cancel [1/2/3/c/d/?] (c): 1
Resolving dependencies...
Resolving package dependencies...
The following 4 packages are going to be downgraded:
net-snmp perl-SNMP sane-backends snmp-mibs
The following NEW package is going to be installed:
libsnmp30
The following package is going to be REMOVED:
libsnmp40
4 packages to downgrade, 1 new, 1 to remove.
Package download size: 4.7 MiB
Package install size change:
| 22.6 MiB required by to be installed packages
-4.8 MiB | - 27.4 MiB released by to be removed packages
Backend: classic_rpmtrans
Continue? [y/n/v/...? shows all options] (y):
Retrieving: snmp-mibs-5.7.3-lp151.7.5.x86_64 (oss) (1/5), 256.3 KiB
Retrieving: snmp-mibs-5.7.3-lp151.7.5.x86_64.rpm .......................................................................................................................................................................................................................[done]
Retrieving: libsnmp30-5.7.3-lp151.7.5.x86_64 (oss) (2/5), 949.7 KiB
Retrieving: libsnmp30-5.7.3-lp151.7.5.x86_64.rpm ...........................................................................................................................................................................................................[done (5.1 MiB/s)]
Retrieving: net-snmp-5.7.3-lp151.7.5.x86_64 (oss) (3/5), 456.7 KiB
Retrieving: net-snmp-5.7.3-lp151.7.5.x86_64.rpm ........................................................................................................................................................................................................................[done]
Retrieving: perl-SNMP-5.7.3-lp151.7.5.x86_64 (oss) (4/5), 136.7 KiB
Retrieving: perl-SNMP-5.7.3-lp151.7.5.x86_64.rpm .......................................................................................................................................................................................................................[done]
Retrieving: sane-backends-1.0.27-lp151.5.3.x86_64 (Plain RPM files cache) (5/5), 3.0 MiB
Checking for file conflicts: ...........................................................................................................................................................................................................................................[done]
(1/6) Installing: snmp-mibs-5.7.3-lp151.7.5.x86_64 .....................................................................................................................................................................................................................[done]
(2/6) Installing: libsnmp30-5.7.3-lp151.7.5.x86_64 .....................................................................................................................................................................................................................[done]
/usr/lib/tmpfiles.d/net-snmp.conf:1: Line references path below legacy directory /var/run/, updating /var/run/net-snmp → /run/net-snmp; please update the tmpfiles.d/ drop-in file accordingly.
(3/6) Installing: net-snmp-5.7.3-lp151.7.5.x86_64 ......................................................................................................................................................................................................................[done]
(4/6) Installing: perl-SNMP-5.7.3-lp151.7.5.x86_64 .....................................................................................................................................................................................................................[done]
(5/6) Installing: sane-backends-1.0.27-lp151.5.3.x86_64 ................................................................................................................................................................................................................[done]
(6/6) Removing: libsnmp40-5.9.4-150600.24.2.1.x86_64 ...................................................................................................................................................................................................................[done]
leaptest:~ #
leaptest:~ # zypper al sane-backends
Specified lock has been successfully added.
leaptest:~ #