New York's "foreign" policy on slavery is probably just as compromised as its ability to reach. Like, obviously no slavery where it can be enforced. But New York isn't going to be picking fights with slavers to liberate things if they don't have to.

A key thing to note is probably its relatively healthy Railroad network. Synths moved between DC and Boston probably often settle there, and I can imagine the New York government giving refuge and shelter for the Railroad while also rooting out Institute covert operations. This would probably extend to "regular" slaves too.

I see New York less as an official body going around freeing slaves but more like a permissive environment for folks like the Railroad or a Followers of the Apocalypse type group to grow and base out of.