It appears that the tags for older release branches have been removed from the MediaWiki repository. This has resulted in broken links (e.g. on https://www.mediawiki.org/wiki/MediaWiki_1.16, which links to https://git.wikimedia.org/log/mediawiki%2Fcore.git/refs%2Fheads%2FREL1_16) and means that it is impossible to find source code for older releases. (Even the release tarballs are difficult to find, which is a separate but related issue - maybe a separate ticket should be logged for that...)
Note that some older releases are accessible via the SVN repository, e.g. https://phabricator.wikimedia.org/diffusion/SVN/browse/tags/REL1_16_0, but this repository only goes up to 1.18. It is also almost completely undiscoverable.
As far as I am aware, these tags take up ~0 bytes in the repository, therefore I don't see why they were deleted. This ticket is to reinstate tags for previous releases so that project history is not lost.
My personal use-case is that I need to support older MediaWiki versions in several of my extensions, and I am finding it increasingly hard to find the information I need in order to know when functions/classes/etc. were introduced. Previously, I could just browse and compare tags.
However, regardless of my use-case, MediaWiki history should not be being obliterated like this - there should be some way for anyone to easily check out or compare any versions of MediaWiki. Ideally this would be within the repository, as this is the simplest to access and to maintain (it is surely less work to keep the tags than to delete them!).
I am open to other suggestions, but I personally can't think of an option that wouldn't be considerably more effort than simply using the built-in Git mechanism for handling this.