I have a large Minecraft server. I do a lot of steps to optimize its.
One of them is to reject attempts to log the banned player. After receiving the first packet, which contains the name check if the name is banned. If the name is banned authorize is not continuing. With a large quantity players this can save some time of server. The new version of Minecraft change name to UUID. I can't check whether the player is banned before authorization. I think this should be changed. The Minecraft client should instead name the player sends UUID to the server.
Sorry for my bad English.
There is nothing to stop them from providing fake UUIDs and making your check useless. This is a feature request and we do not see any gain from adding it, sorry.
If you want to throttle logins, do it by IP.