Another POV: Slack has basically killed forums. It progressed the symbiotic problem forums had vs social media groups 3-4 years ago:
1. Slack is incredibly easy. It was designed to replace mundane emails / bloated inboxes, which when you think about it, is a pretty solid parallel to a forum thread. It did that and more. You can join an infinite number of teams, so with the app you have a single-sign on experience no web platform can rival.
2. The tech crowd has embraced it because of the the above: If you want to contribute to a framework/platform like Bootstrap or React, or you've signed up for an online course or workshop, you don't join a private forum, you just apply to the slack team.
This create a paradox for a platform like this: Old member have left the platform and will be unlikely to return; and new members will be dissuaded to join based on that missing presence.
I'm going to extend this, actually: Forum problems have been compounded by the above plus professional growth. By that I mean you either outgrew the platform, or it outgrew you.
When you were learning, you were just screwing around. Trying to see if design or dev was actually for you. You tried bad affects, bad UX decisions, bad coding techniques, bad plugins, etc. You were just trying to figure out what works and what doesn't.
You outgrew the platform
You figured it out and transitioned towards doing commissioned work, which is doing a specific piece for a specific use. For legal reasons you can't just show off WIP and you become less active. A market place becomes useless to you because you're selling a service, not a product, which is contextual and subjective – IE I was commissioned to design a hockey uniform for a US Under-18 team a few years back, because I love hockey and I understood the culture. I'd never be able to do another sport, or even a European hockey team because of culture - ad space, etc.
The platform outgrew you
You figure it out and now you have an inventory of designs/templates you want to resell over and over again. Forums aren't a marketplace, they're ultimately about discussions, so eventually the novelty of selling a template wears off and you leave it for ThemeForest or CodeCanyon. Then npm came along and undercut everyone trying to sell code snippets.