Is your feature request related to a problem? Please describe.
There is currently an ongoing problem where people [un]intentionally join servers which have ViaFabric in them with modifications that purposely add new blocks/items while at the same time allowing the client to cancel the vanilla contents outright in favor of it's own ones which amendments does for example.

Is this related specifically to ViaFabric?

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

Describe the solution you'd like
We should attempt to registry match the server and client in case the client happens to have mods that destroy the mappings system as result.

Describe alternatives you've considered
Blacklist the modifications instead which is simpler but in reality is not worth it as the list would then append very rapidly.

Which versions is this for?
All of them.