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?
No, people literally still roll their own multiplayer networking in 2023. Seriously. Unreal Engine 5's out-of-the-box multiplayer code isn't production ready, and they're probably the "best" of the major engines people use today.
It pales in comparison to older engines from other game companies. It's effectively unusable.
This seems wild in the free-to-play era, where multiplayer with in game purchases is, like, the main business model. Maybe Unreal figures the multiplayer implementation is the metric on which the game studios compete, so there’s no hope to commoditizing it, or something like that?
It's also why so many multiplayer games are bad. Wanna know what's worse? Unreal Engine's architecture is unreliable by design.[1]
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.
