Some relationships ended because of slowly building resentment that dramatically erupted, or because someone was mistreated, or because I realized the person was just a total dickface to begin with. I have no interest in maintaining a friendship with those ex's.

Other relationships ended because we just weren't compatible romantically, realized we has mismatched values or long-term goals, or were never meant to be more than a short-term fling in the first place. Sometimes I have reconnected with these folks and maintained friendships, always after a period of no-contact so that we could both heal and move on emotionally and neither person has lingering feelings for the other beyond a general nostalgic fondness.

There's probably a zillion shades of situations in between, and it's different for every pair of people. Hard and fast rules like being friends with every ex by default (or never any ex ever) don't really apply. It's a case-by-case basis sorta thing as long as there is trust, transparency, and open communication in the current relationship.