One thing about these old school forums is they are something you host yourself (directly or paying a server somewhere), and this requires but knowledge on doing so, and time to do its maintenance (beyond moderation and stuff). Additionally, I don’t think simple machines and phpbb development has kept as strong as the people trying to spam it.

Discorse is probably the best forum software these days. It's incredibly nice and works just as well on mobile and desktop.

phpbb was a nightmare to keep patched (even without any plugins/customization) and free of spam accounts back when forums were in their heyday and spamming wasn’t even that lucrative. 20 years ago a fresh install via cpanel and getting indexed would be enough to have hundreds of accounts being registered just for SEO juice from the homepage URL field in user profile pages.

I can only imagine how infinitely worse things must be now.