Explicit Sync, yes. The reason why I consider previous implicit sync a bug is that it was essentially unfounded expectation - nothing guaranteed you'd get a magic mutex in the sky that synchronized renders for you, and in fact, you do not want one for maximum performance. Lack of explicit sync also essentially meant common multi-gpu setups would be broken.

Androids graphics stack and drivers also use explicit sync, it isn't just a Nvidia thing.

I was not saying it's an nvidia thing, just that significant majority of devices used by wayland proponents had undeclared implicit sync issues that were invalidated when faced with a driver that didn't have them.