I am not sure simply avoiding conflict (unless in an unhealthy way) is a sign of immaturity. Many people don't like conflict, and that's mature. Why would we want our lives plagued by it and constant arguments, etc. Nobody would want that.

Disregarding boundaries could be a sign of immaturity, provided boundaries are set in the first place or aren't dysfunctional. Dysfunctional boundaries are always a problem, whereas health boundaries are freeing and flexible. We welcome people into our space who treat us with respect and kindness, not those who abuse us. That's healthy boundaries.

Feedback is an issue when it's given inappropriately or in an unclear way, or damaging way, so it's not constructive feedback. Feedback should be free of any kind of manipulation or selfish behaviour, and not seek to control the other person's narrative. Such feedback isn't constructive and causes damage. Not accepting that kind of feedback is healthy. Accepting positive constructive feedback on the other hand is a sign of growth, which few immature people accept. So fundamentally, not accepting constructive feedback is a sign of immaturity, but the nature of the feedback should be genuine, with the intention of helping the other person grow. If it's a personal attack on someone else, with a view to controlling how others perceive the other person, that's absolutely terrible and is nowhere near constructive or positive feedback. It's passive-aggressive and unhealthy.