I've recently been setting up a velocity server network for some friends and friends of friends, and I agree with your findings. I don't have much history on Forge vs Paper vs Fabric vs..... (and found it all very overwhelming, honestly) but from what I can tell, the popular sites like modrinth have communities way more focused around Forge/NeoForge.

Paper does seem to have it's own site for plugins, hangar or something? (Don't have my web history on this PC) but the community support doesn't seem nearly as fleshed out.

It is incredible though, before 1.21 the last time I played around with MC server hosting was probably around 1.8 days, when it seemed like you only had Bukkit and a few plugins for it