So, I've seen a very similar comment on this matter, and wanted to see what others think, as well as share my thoughts.
So, originally bill cipher couldn't leave gravity falls because there was a unicorn hair barrier in place. Regarding that, i have a few questions about it, as well as theories as to how bill could escape.
First off, the unicorn hair barrier must be either hidden, or protected. So there are 2 possibilities i thought of as to how they are secure:
A) The unicorn hair is buried
B) The unicorn hair is hidden in plain sight (there is either some kind of illusion that makes it look like a powerline, track, path, etc., or a spell that makes it invisible)
Second, bill cipher.
I think he is alive, either he is in stans mind, and is trapped in there (which theoretically means stan shouldn't be able to pass the unicorn barrier, and leave gravity falls, right?)
Or he has taken up a different form, in another dimension. Like maybe he has turned human? (Alex Hirsch, PLEASE make it canon😂)
He might also be trapped in the statue, waiting for someone to shake his hand. Then he can bargain to be freed?
He might also be waiting for the next time to be summoned
Last but not least, referring to how bill could escape gravity falls,
1) Bill cipher, while in gravity falls, was able to fully teleport. Why couldn't he teleport outside of the portal, or teleport to a different dimension/planet/universe/galaxy, and from there teleport to a different place on the planet?
2) He could've made a hole through the planet ( Does that make sense? And just fly to the other side of the planet. No wait, would the barrier get in the way?
3) Couldnt he have possessed someone, or "borrowed" someone's body to leave? Wait, would the barrier interfere?
4) He could've used the mindscape, and made/forced (whichever term you prefer) them to move the unicorn hair? Couldnt he? It would have been awfully easy for him to find. He just needs to go in one direction until he smashes into a wall-like barrier.
I hope you enjoyed reading my book 😂