This plugin provides an improved version of the in-built whitelist system:

• it works for Floodgate (Geyser) users
• allows you to customize the whitelist messages and more
• is well maintained

This plugin depends on CommandAPI (https://www.spigotmc.org/resources/api-commandapi-1-16-5-1-17-1.62353).

You can find a more in-depth description in the README file on the plugin's repository:
https://github.com/MatsG23/UniversalWhitelist/blob/master/README.md