[[ https://www.mediawiki.org/wiki/Extension:CheckUser | CheckUser ]] (#mediawiki-extensions-CheckUser · rECHU) is probably one of the oldest extensions still working in Wikimedia Projects.
Designed c.2005, the extension helps to deal with problematic cases of abuse such as sock puppetry, vandalism and spam ([[ https://meta.wikimedia.org/wiki/History_of_user_rights#CheckUser | some history here ]]). As time goes by, the needs of the projects increased, and as we can see in the [[ https://phabricator.wikimedia.org/project/view/203/ | CheckUser workboard ]], the bugs accumulate without being resolved, primary because its code is so old that working on it is a true pain (refer to {T132892} and it's related tasks, as well as the workboard I've linked above).
The lack of an active mantainer of the Extension is also problematic in this case. To resolve this issues, I think we need a new CheckUser extension which can work with all the new features and the new code MediaWiki has at its current state. And we can also take this oportunity to gather opinions from CheckUsers on which new functions the new CheckUser extension should have, etc.
Sorry if the format is wrong or I missed something. It's the first RFC I fill here. Let me know if there's something that needs fixing and I'll try to do that.
Best regards.