Specialized forums certainly still exist, but according to my perception, the activity on many of them has decreased A LOT compared to 10-15 years ago.

People spend much more time on FB, Instagram et al.

Reddit is an interesting hybrid. it does allow for specialized communities, but still engages in the "content feeding" mentioned above.

In forums, deep discussions can/could go on for weeks on the same topic. This completely impossible in today's main platforms.

Hackernews is just as guilty of this!

Few people read submissions older than a day or two, and a miniscule amount of them comment. Because no-one is going to answer anyway. That's my biggest gripe with this community.