If I strike up a conversation with you, and suddenly I don’t want to talk to you anymore, then what, if any, obligations do I have? Can I just stop talking mid-sentence and begin to completely ignore you? After all, I did not promise you a complete conversation.
Or did I, by engaging in a civil conversation with you, implicitly promise to abide by the normal social rules of etiquette, as far as I am reasonably able?
It is similar with software. If you, say, put up a web site (or even just a README.md) containing blurbs about how useful your software is, extolling its virtues, you are implicitly promising future updates and support, to the best of your (limited) ability. If you need to step away from the project, you are expected to do so in an orderly fashion (again, to the best of your – possibly limited – ability), announce it publicly, etc.
If you have no web site, but you have given similar indications in conversations, the same principle is applicable, but you have fewer people to notify.
> Who get to decide where the line is?
If a user can reasonably feel let down by your actions, or can reasonably feel that you have misled them, then I feel that a line has been crossed.