ViaVersion
Allows the connection of newer clients to older server versions
...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 or through ViaFabricPlus due to having direct access to client/server state and more extensive API.