ViaVersion
Allows the connection of newer clients to older server versions
Allows the connection of higher client versions to lower server versions - it works on any platform you can think of. The base ViaVersion jar runs on Paper and Velocity. We also have projects integrating ViaVersion to run on Fabric, Forge, Bungee, Sponge, or as a standalone proxy to join from basically any client version on any server version from the past decade. See HERE for an overview of the different Via* projects. Note that ViaVersion will be able to run best on either Paper servers...