Current extension repositories have REL1_23 branch, but it was created when REL1_22 was created too, so basically it's outdated.
Examples:
https://git.wikimedia.org/branches/mediawiki%2Fextensions%2FCite.git
https://git.wikimedia.org/branches/mediawiki%2Fextensions%2FImageMap.git
https://git.wikimedia.org/branches/mediawiki%2Fextensions%2FCheckUser.git
They have REL1_23 updated as of 22-10-2013
I've only found one exception, though:
https://git.wikimedia.org/branches/mediawiki%2Fextensions%2FWidgets.git
One particular change introduced on 1.23 release was the migration from PHP to JSON files for i18n. I think all the extensions were migrated to JSON files in master, but REL1_23 still doesn't have that change.
Version: 1.23.0
Severity: major
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=67270