https://gathering.tweakers.net (mainly tech but also general discussion) is still huge in the Netherlands. And https://boards.ie (general) is in Ireland. In Spain I know https://forocoches.com (cars) is big. https://xda-developers.com is the absolute authority on Android ROMs. They're not all gone.

The time that everyone started a forum is gone. Many smaller ones have shut down. But some are still thriving.