I am kind of surprised no-one has mentioned the obvious: Hacker News. Unless I've misunderstood your question, the bulk of web dev discussion happens in technical posts on personal and business blogs, which are then aggregated right here. It's a big part of why I'm on here.

If you're talking more about chat, the more messy "pair programming" side of web dev, I have always found this happens in actual dev teams who are working on the same product or for the same business. You do absolutely get chat like this at conventions - I have been to DjangoCon and PyCon back in the day and there were enormously useful discussions at those - but devs need to have something in common to talk about. As someone else has said here already, web dev is a far far broader topic than you might think - I have often found speaking to other devs I did not understand what it was they were doing. Alberta Tech did one on this: https://www.instagram.com/reel/DBSpm2CNuGF/?igsh=NGttZzk5NzB...