Multiplayer WorldBorderhaul allows for each player on a server to have their own world border with varrying settings
Gamerules:
World Border On - Toggles whether or not the world border is on
Starting World Border Size - The Radius of the world border at start
Grow With Advancements - Toggles whether or not the world border grows with every advancement gained
World Border Growth - The amount of blocks the border will grow with each advancement
World Border Games - Toggles whether or not each player will be spread out with individual world borders
Commands:
/border invite -player- -invites another player to your world border
/border accept -accepts another players invite putting you into their border
/border return -returns you to your world border

