Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Is this related specifically to ViaFabric?
(Optional)

• It seems to not apply for ViaVersion/ViaBackwards/ViaRewind installed on Bukkit/BungeeCord/Sponge/Velocity servers.

Describe the solution you'd like
A version for 1.12.2 forge

Describe alternatives you've considered
none

Which versions is this for?
1.12.x

You want to connect to an older version than yours, or a newer one?
If you want to connect to an older version (ex: 1.9), you can already use https://github.com/5zig/The-5zig-Mod/releases/tag/3.12.5 and add https://github.com/Gerrygames/ClientViaVersion/releases/tag/v2.0.8 in the plugins folder of 5zig.

I was about to say; that mod is the only thing available in this regard currently. Making Via support forge without modifying forge behavior is next to impossible and thus not planned or a sought-after endeavor. Especially given that any kind of mod that modifies the vanilla game in any sort of meaningful way is gonna break Via support.
Since Via* is all open source though, you may create that kind of mod yourself if you so desire.

See https://www.curseforge.com/minecraft/mc-mods/viaforge