An excerpt of the file RELEASE-NOTES-1.22 that ships with 1.22.0 tarball:
MediaWiki 1.22
THIS IS NOT A RELEASE YET
MediaWiki 1.22 is an alpha-quality branch and is not recommended for use in
production.
Of course we want to change that :)
Version: 1.22.0
Severity: major
URL: https://git.wikimedia.org/blob/mediawiki%2Fcore.git/REL1_22/RELEASE-NOTES-1.22