ViaVersion and ViaBackwards serve different aspects of connectivity. Determine first whether the client version is newer or older than the server.
Step by step
-
Write the server version and client versions you will allow. For newer Java clients, check out ViaVersion's support range.
-
If you want to accept older clients, check out ViaBackwards' support table. Install the required ViaVersion dependency in the compatible version.
-
Log in with the basic server version and a sample client from both directions. Custom content or inventory behavior may vary by version.
-
Write the working versions to the server's connection page. Don't promise a general release range to an unsupported client.
Keep in mind
There may be separate compatibility limits for very old protocols or modpacks.