I was doing an update on my wife’s Leap 15.4 laptop today which included the latest kernel default. The install failed because it said there was not 25mb available on the boot partition, however there was well over 25mb free on it! I installed everything else from the zipper up and retried the kernel install from the YAST gui but it still failed for the same reason.
In that case the message is not really that clear and it should be more precise and say 25mb more than the available space. Also why does the installer accept a boot partition without warning if it is not recommended?
Because as i already tried to explain in some special cases it makes sense to have a boot partition (dual boot with Windows, RAID, LVM, …). Modern Linux distributions don’t create a separate boot partition anymore except you chose to use custom partitioning. The /boot directory is a part of the single system partition nowadays.
Here is a good explanation of advantages/disadvantages and use cases of a separate boot partition: https://www.baeldung.com/linux/boot-partition-necessary
But it is important not to confuse separate boot partition and /boot/efi…