Is your feature request related to a problem? Please describe.
When using the client-side mode to play on specific servers such as 1.8.x; some blocks/items don't play nicely with client-side mode alone or related,

and since viaversion itself makes an attempt to translate clients in order to aid players through snow and whatnot, it allows server owners to ask the plugin itself to try and patch such issues to make gameplay properly work in some cases.

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
Allow the client-side mode to simulate the changes on 3rd-party servers. (rather than just have it already configured on specific servers or singleplayer with LAN enabled)

Describe alternatives you've considered
Use multiconnect as an extra mod to try and help out with the problems, although this is known to cause further problems if compatibility issues can occur and whatnot but it's still worth the alternative if viafabric fails the main job.

Which versions is this for?
The latest version of ViaFabric at time of writing this comment.

Something i did not add in this issue; Currently, you also cannot blacklist versions (such as 1.9.x, 1.10.x, etc) on viafabric's client-side mode as it will make use of them anyways when enforcing versions yourself via config.

Already possible in older clients, therefore this is no longer relevant.