Is it inappropriate? Not really. Is it a threat? Yeah, it really is. “I’m going to stop using your software if you don’t change this” is a threat. “I’m going to fork it and ship a competing product” is a stronger threat.
It was a clear attempt to sway with intimidation. It was done politely but that doesn’t change what it was. (The dev had already closed as won’t fix, too, so they had preemptively answered.)
“hey, X is a dealbreaker for me, if this is not fixed please let me know so I can look for someone else”
“how dare you threaten me?”
I understand how one might feel attacked by an ultimatum, but a policy of rejecting ultimatums altogether only has the benefit of keeping pride intact, on the other hand it discourages people in your life from giving you a last chance, from communicating why they are leaving, and for letting them come back if the parting was amicable
best response is to just communicate you are not fixing X and parting ways amicably, no hard feelings. not only is it beneficial for the ultimated party, but it’s fair for the ultimating party, who is absolutely in their own right to rescind their voluntary relationship, in no way is it a threat to communicate that possibility, because a threat implies violence or damage.