Codesearch had a few hidden backends, one for the super legacy ArmchairGM repository (rAMGM ArmchairGM) and another for the ShoutHow project (research project for improving wikiHow-authored code), accessible originally at https://codesearch.wmcloud.org/<project name in lowercase> ; as of today both URLs throw a "Unknown search backend <name>" error.

However, looking at the app.py file on the labs/codesearch repo as of current master, both hidden backends are still listed there, so surely they should work?! Yet they...don't.

FWIW, I had requested the ArmchairGM repo to be added in 2018 in T188536 and it's still somewhat semi-useful in Social-Tools development for figuring out stuff about how something came to be; that said, it indeed is an archived repo so it's totally reasonable for it to remain a hidden backend because the various extensions (SocialProfile, MediaWiki-extensions-Comments, VoteNY, etc.) which are maintained have their own repos on gerrit.

The ShoutHow project could, should someone want to do that, be visible for it is a project that's supposed to be maintained and usable and based on a currently supported (albeit legacy -- 1.35) MW upstream release, and I certainly do accept patches etc. but since it's not hosted on gerrit or github, account creation is the tricky part. I have no strong opinion on that either way, that's hardly the issue, I thought I'd just mention that though both were and are hidden from the UI, only one of them is an obsolete archive 99.9% of people are unlike to care about.