I am updating an old wiki in 1.20 and set up a dev environment to prepare the update, and my git clone -b REL1_20 https://gerrit.wikimedia.org/r/p/mediawiki/core.git returned an error, relatedly to T92503.
I aggree with the spirit of T92503 (remove old branches), but it could be created Git tags named REL1_XX to keep compatibility with the old behaviour: git clone -b REL1_20 https://gerrit.wikimedia.org/r/p/mediawiki/core.git would work.
Even if old versions are not supposed to be used in normal temperature and pressure, such tags could help to smooth transitions from active branches to historical "branches". Without it, it would be more difficult to get the last minor version of some branch: you have to git-clone the whole repository, list the tags, find the latest tag in the branch, and git-checkout this tag.
This solution was evoked in T179754, this task is dedicated to this and not directly linked to SemanticMediaWiki’s CI.