This one actually gets use. I bumped up against an error in our internal services and it was returning 420's... Not fun to diagnose
Don’t give Elon any ideas
I think twitter is where it was first used, actually. They stopped after a while, but from what I read, it was a code you got before hitting your API limit. (iirc, you got a 417/429 error when you ran out)
HTTP 527 Railgun Listener to Origin
That one's not standard, and probably shouldn't ever have been used in the first place IMO