Vsync increases input latency since it works by buffering frames (2-3 usually). In some games this can be even more severe due to less than ideal implementations.Metal generally enforces Vsync. I think it can be disabled on certain conditions, but I'm not even sure it's possible to disable Vsync with Metal on Apple GPUs. AFAIK, it's impossible on iPhone and iPads, but I could be wrong.
Who needs to disable it anyway? Screen tearing is just awful and with Promotion, there is really no point in disabling Vsync. It's just useful for benchmarking. But recent built-in benchmark tools have means to estimate frame rates with Vsync on.
Additionally, non-adaptive vsync will need to sync to some multiple of your displays framerate, so if you for example manage 40fps then non-adaptive vsync will force you to 30fps.