MediaWiki extensions have two basic options for releasing: one (called rel) is to have a separate branch for each major[^1] version of core; the other (called master) is to only have one branch and to make it compatible with as many versions of core that the maintainer wants (hopefully all current versions). Extensions also have a version number of their own, and for years it’s confused me. I think that it doesn’t make sense to have a version number with a rel compatibility policy. It does make sense for master (and in that case, the automatically-created REL1_* branches should be deleted).

[^1]: note that MediaWiki doesn’t use Semantic Versioning, so it’s not the first number; that’s always 1.