Is your feature request related to a problem? Please describe.
so 1.21.1 is supported for viabackwards but not for viafabric but viabackwards needs viafabric to work

Is this related specifically to ViaFabric?

Describe the solution you'd like
add 1.21.1 to viafabric

Describe alternatives you've considered
none

Which versions is this for?
1.21.1

Will not be done as 1.21.4 has superseded it including 1.21.3, but older builds of viafabric are available as a workaround but you will need to include latest viaversion jar.