Velocity Proxy
Velocity is a Proxy software developed by the Paper Team as a fast and modern proxy software, intended to replace Waterfall, which was itself intended to replace BungeeCord.
It has it's own plugin ecosystem separate from BungeeCord/Waterfall, so if you're looking to migrate an existing proxy, you'll need to find alternatives for plugins that don't support Velocity.
It's under active development after the deprecation of Waterfall, and is the recommended replacement for Waterfall. You can download Velocity from it's download page on Paper's website.
Plugins
Just like with Bungee/Waterfall, plugins for Spigot/Paper won't work on Velocity, but Bungee/Waterfall plugins won't work on Velocity either, so unless the plugins support both Bungee and Velocity, you'll need to find alternatives that do support Velocity. You can find plugins that support Velocity in the following locations:
Configuration
Paper has very extensive documentation on using Velocity, we have a few links to some important sections below, but encourage you to explore on your own as well.
Securing
We don't offer IP whitelisting or firewalls for our game servers, so you'll need to make use of Velocity modern forwarding to secure your backend servers on your Velocity Network. You can find information on that method as well as related advice on the Velocity Docs.