Is your feature request related to a problem? Please describe.
I use some mods that have custom blocks, for example, if you install https://www.curseforge.com/minecraft/mc-mods/the-wild-mod then you will have blocks such as twm:sculk_catalyst, but if I modify mappings-1.18to1.19.json and change it to "minecraft:sculk_catalyst": "twm:sculk_catalyst", it gives me error that the mapping for minecraft:sculk_catalyst does not exist and if I join 1.19 server with 1.17.1 it shows me nothing (but cannot pass through since it's a solid block) where there should be a sculk catalyst.

Is this related specifically to ViaFabric?
(Optional)

• It seems to not apply for ViaVersion/ViaBackwards/ViaRewind installed on Bukkit/BungeeCord/Sponge/Velocity servers.
  ------> in servers there is a different error that that kind of block does not exist but i'm not doing this in servers.

Describe the solution you'd like
A clear and concise description of what you want to happen.

• Fallback block mappings can use custom blocks in other fabric mods

The mapping files work by getting the index of both blocks from the respective mapping-<version>.json from https://github.com/ViaVersion/Mappings/tree/main/mappings, so twm:sculk_catalyst also needs to be in one of them for the mapping generator to work.

Custom content (like blocks/entities) are also not really supported by us.