I've just been browsing https://releases.wikimedia.org/mediawiki/ and it appears that there are big holes in the history of MediaWiki.
For example, the 1.19 folder includes 1.19.9 to 1.19.24 but is missing 1.19.0 to 1.19.8, and possibly one or more beta releases. Similarly the 1.20 folder only contains release 1.20.8. (Note that I have only checked these two folders, so I don't know how much further this problem extends.)
Expected behaviour: The release archive contains an archive of all releases.
I am not sure if this is corruption (e.g. during server moves) or whether it was by deliberate user action, but these deleted versions should be restored and should remain publicly available.
See also, T190363, which relates to properly tagging the releases in Git.