I was researching this recently (why, I'll say in a sec), found a couple of interesting clack channels of sites people mentioned here: producthq, women in product, product coalition, product school, etc..

A lot of discussions are unstructured or hard to keep track of. I mean, it's slack, not a forum.

What I'm helping create with the wonderful team at JAM London is a curated collection of 'Hot Tips' with short practical and no-nonsense answers form PMs for PMs. It's picking up interest, looks like your question hit the nail on the head! :D https://www.jamlondon.io/tips/