Is there any good template/library for modern multiplayer netcode for Unreal or Unity or anything else, or is it truly everybody rolls their own as the secret sauce in their AAA games?
It pales in comparison to older engines from other game companies. It's effectively unusable.
Basically, Tim Sweeney's approach for Actor movement wasn't to replicate state accurately, but just trust that clients would eventually get the right data. This isn't the same as eventual consistency mind you, because game state constantly changes in fast-paced games.
You can have whole time spans where Actors have absolutely no accurate player position replication.
[1]: https://docs.google.com/document/d/1KGLbEfHsWANTTgUqfK6rkpFY...