While looking for an answer to my question, I found an old post (from 2015) about encrypted partitions that I have an answer for; but the thread is closed. Can I post my answer to it, or should I open a new thread (which will not be a question) and link back to the closed thread?
The latter option is your only possibility. In any case, it is best not to tack on to old threads (even when they are open), as quite often things have changed significantly anyway.
Further to deano_ferrari’s comment, when you start a new thread, you can always reference the old thread with a link if you believe that is useful (with appropriate caveats).
And further, think about the effect of such a post. Not many people that participated so many years ago will still be subscribed to the thread and thus get no mail about your new post. And how many people will look for posts added at the end of such an old thread? More chance that they look for new threads that are started to see if there is anything that interests them.
Remember that five years is a long, long time in the computer software world. All versions have changed. Not many people will have a clear idea about how it was five years ago. And indeed we generally close old threads that were interesting in an old version exactly because they are about old, not supported versions (and to have less bait for forums spammers ;)).