We don't use this extension in production, and it's not really maintained upstream. It uses a bunch of removed functions which mean it won't work in 1.27.
We don't use this extension in production
This is correct, but that's also because we don't have that many overly technical wikis, and also because we've never advertised it. Granted, that hasn't stopped users in the past from requesting different kinds of unsupported extensions, but still.
and it's not really maintained upstream.
Really? This is news to me, given that I'm the one who volunteered to maintain it on upstream. If I haven't commented on something CodeReview-related, it's because I wasn't cc'd to it -- I'm more than interested in CR-related matters!
It uses a bunch of removed functions which mean it won't work in 1.27.
What functions? Are these tracked on upstream? If not, can you submit tickets? I'm willing to look into the issues and fix them to make CodeReview 1.27-compatible. (Somewhat relatedly, once the issues are fixed, we could probably set CodeReview as an external because it's queued to be decommissioned from WMF prod and even if that wouldn't happen, it's extremely unlikely to be broken by WMF developers. The same can't be said for popular, actively developed extensions like Echo, for example.)