multiconnect

Allows a client on the latest Minecraft version to connect to servers on older versions (as well as the current one).

Currently goes back to 1.11 (stable) and 1.8 (experimental).
Requires Fabric-loader

Why another protocol translator?

Multiconnect distinguishes itself from similar projects such as ViaVersion and ProtocolSupport in that it is very much client-side. Crucially, this means it can (and does) fix many version differences on the client; such issues are unfixable on the server. As of the time of writing, multiconnect is the only mod which fixes 1.12 swimming mechanics, parkour differences, and re-adds removed features such as command syntax on the client.
On the other hand, multiconnect supports only the latest Minecraft client version, and only on Fabric. It also does not have as much version coverage as ViaVersion and ProtocolSupport (yet).

Usage with ViaFabric

If you want to connect to servers older than what multiconnect can currently support, you can install ViaFabric alongside multiconnect, the two are compatible - multiconnect will translate as much as it can, ViaFabric will take you the rest of the way. Follow the instructions in the ViaFabric readme for details on how to do this.