I believe Vermont was the first, period (unless you count the Somerset case in Britain). Though slavery had died out in various other places before that without a legal ban, and had been limited by laws in various places & ways before 1777.https://twitter.com/Tracinski/status/1231690062448021510 …
Yeah, there was real uncertainty about its impact at the time. It did, ultimately, make it legally impossible to hold slaves inside Britain because Parliament didn't step in to overturn it.