I have some doubts that, on most human-relationship topics, anyone knows their preferences well enough to correctly identify such turning points, whether expressed as ultimatum to others (if you don't quit smoking, I won't be around you), or only as bright-line test for themselves (if they keep smoking, I'll be happier without them).

This uncertainty makes it very common for an ultimatum to be given as a negotiating point, rather than a true prediction of conditional events. This is especially true for something like a Schelling fence on a continuum - getting yourself to commit (and your partner to believe) to the unpleasant action if an arbitrary line is crossed is not easy, and for many people on many topics, not possible.