Gregory's Server
It's official now: Threads implemented FEP-e232
https://engineering.fb.com/2024/03/21/networking-traffic/threads-has-entered-the-fediverse/
Article contains a couple of minor inaccuracies: _misskey_quote doesn't build on FEP-e232, and the other property name is quoteUrl, not quoteURL. But overall it's good and I hope other implementers will notice it.