This mod for Minecraft Forge provides customizable login messages.

Usage examples:

/loginmessage add Welcome to the server!

/loginmessage list

/loginmessage remove 1

/loginmessage removeall

/loginmessage help

/loginmessage add <<c>>Red text

/loginmessage add Hello <<playername>>!

Project pages:

https://modrinth.com/mod/custom-login-messages

https://www.curseforge.com/minecraft/mc-mods/miraisoftloginmessages