Is it specifically a Bosmer-homelands thing?
If a person of another Race went into the Valenwood and then violated the Green, it wouldn't matter cause the Pact doesn't apply to them. A Bosmer, maybe, but that's up to the judgement of the locals whether or not "outsider" Bosmer should be treated under the Pact.
Its real - it also had something to do with Molag Bal sacking that Bosmer settlement that you save in one of the quests.. Just googling it.. Gil-Var-Delle.
Many a historical fiction piece has attempted to dramatize what occurred there, but none of those stories can be confirmed, except to say that a catastrophic event struck the town. Perhaps the residents were killed, perhaps they fled. None were ever heard from again, but for all anyone knows, a large fire could have been the culprit. I can't imagine anything more catastrophic than that to a Wood Elf dwelling.
There are references to Bosmer who are "excused" from the Pact (e.g. Vinedusk Rangers), and those who said they didn't grow up with that tradition (some of the Hireling letters?), and those who didn't want any part of it (early Bosmer shapeshifters).
Are all these Bosmer kidding themselves and are gonna suffer in Ouze hell? Or are some actually allowed to not participate in the Green Pact? Is it specifically a Bosmer-homelands thing?
Maybe not any mystical consequences, but I'm pretty sure some hard core Bosmer enforce it against other race visitors.
But that's probably in the extreme.
So a Bosmer in a small village way out in the wilds of Malabal Tor will likely be more devout, be surrounded by those who are also devout and because its closer to nature be impacted by the Pact more. So when a Bosmer breaks the Pact in that circumstance, the consequences are more swift and immediate.
But this is more like "legal consequence". What about mystical consequence? Does Y'ffre care about how far you moved away from Valenwood? Are you still going to end up in that Ouze hell for violators because the existing Bosmer are descendents of the ones who accepted Y'ffre's deal way back when -- unless somehow not all of them are?