Have you ever heard of a person forming a polyfidelitous relationship with their childhood/long-time friends?
Try asking yourself: why does it matter whether I’ve heard of this situation? I’m just one person! Lots of things happen in the world every day that I’m not present to.
Deep love between lifelong friends is very common and well documented. I can’t point you to a specific memoir or anecdote from my life that covers this this exact situation, but again, it really doesn’t matter whether my experience encompasses something.
If this is a situation you’re exploring and you are looking for advice or mentorship from people who’ve walked this road before, try checking out other polyamorous resources and communities. I’m pretty sure most of the advice out there about polyamory applies to this situation. If you have a specific situation you want advice about that’s unique to a polyfidelitous relationship formed by childhood or long-time friends, send that specific question in! If you just want validation that it’s a thing that has happened and can happened, don’t worry about it.