Always fun to continue to see innovation within the Minecraft Java space.
At Hypixel we ran (and I believe they still do run) a custom fork of Spigot from 2014-ish, with features from each sequential update to Minecraft being added to our fork via our Spigot fork. This let us diverge greatly and customize the Minecraft protocol to our own needs, saving hugely on internal bandwidth and letting us optimize crap out of the L7 "BungeeCord" reverse-proxy we used.