It has to do with a shift in culture I think. When gmod was in it's golden years you could easily pop in and make friends because that's how the internet was years ago, but now I feel as though people are more in it for money or are trying to gatekeep new people.
The same could be said with most multiplayer games though as I've notices a lot of people have the mindset that it's their game and if they don't know you they don't want you in. That and communities are typically grown from places places like Discord rather than just in-game like they were back then. Even IRC and mumble were places where people hung out but friendships were still created in-game for the most part.
The culture and userbase has changed over the years but I've been able to find quite a few welcoming small servers that were fun. That said, I hope when S&box comes out that it garners that old gmod style of community, but here's hoping. The closest experience to the old gmod style of community in a new game would be VRChat and NeosVR as those have a lot of spaces to make friends and play games all within the game itself.