It kinda depends on what you’re looking for. There are plenty of forums where merchants ask a lot of questions, however if you’re looking to go into a discussion with ppc specialists it’s rough. I’d almost say that reddit is quite active in this industry. You can try blackhatworld there is an active user base in the ppc section. From my experience most advanced ppc guys hang out in slack channels or have their monthly meetups with other contacts from other agencies for example.
I think there is little open space for advanced discussions because it’s quite a niche and a lot of talk often stays inside an agency or a company in terms of strategy or best practices other than regular pitch material.