It's complicated and a deeply personal decision. In any long term relationship, romantic or otherwise, people will likely hurt us in some way. If you choose to run to your friends/reddit in that moment, the advice you get is 'leave', because the hurt is all they see. It's a well-meaning, but ultimately harmful knee-jerk reaction, because it lacks nuance and perspective.
Boundaries come in many forms. Cutting someone out is for when there is a total breakdown in the relationship, you just don't see a way forward, or you lost any interest in trying. There are softer ways to go about things, if you are unhappy with someone's behavior but don't want to burn the bridge entirely. Letting the person know you will be taking some space. Directly telling and showing them you won't put up with certain bad behaviors, and giving them a chance to course correct. Talking vulnerably about why they hurt you and what you need to be done differently. Or giving them a lesser role in your life than before. All of these are options and many more, depending on context. Communication is key, as well as you speaking and demonstrating your boundaries.
To answer your question directly, here are my very personal and subjective criteria to leave and to stay. Though real life is messier and more complicated, so it does depend very much on context. For example, I will generally be more flexible around people I have built up meaningful history with, over passing friendships, and of course family gets a lot more forgiveness and second chances.
Generally, leave them behind if:
Stay if: - This is a rough patch in an otherwise solid long-term relationship of any kind, the hurt was done thoughtlessly or in a flare up. - There is potential to communicate and come to an understanding. - There are softer boundaries to deal with the problem than outright leaving. - The hurt is a bug, not a feature.