This page says it has, as does the email from Chad on MediaWiki-announce. However, Release notes/1.31 says it is not a release yet and is not recommended for use in production and Special:ExtensionDistributor still shows 1.30 as the latest stable release with 1.31 as the next stable candidate. I presume the answer is that 1.31 has indeed been released but inconsistent messaging doesn't give confidence. I won't complete my upgrade until I am confident that ExtensionDistributor is giving me the correct version of each extension.
Topic on Talk:MediaWiki 1.31
Yes.
I've re-copied https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/REL1_31/RELEASE-NOTES-1.31 to the wiki page (though I don't really understand what the purpose of that page is).
Bumping ED is going to happen in a few minutes' time: https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/440283
Also the git repository is still showing RC2.
https://phabricator.wikimedia.org/source/mediawiki/browse/REL1_31/includes/DefaultSettings.php
The link you gave is to Phabricator's mirrors which sometimes lag a little. The actual git repo is on gerrit, and as shown https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/REL1_31/includes/DefaultSettings.php#74 this is not out of date there.
File: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/REL1_31/RELEASE-NOTES-1.31
Shows:
THIS IS NOT A RELEASE YET!