Fabric doesn't try to check if mods are compatible with vanilla clients and don't remap the registry IDs for vanilla clients
Reproduction steps:

• Start a server with 19w11b with Fabric API only
• Upgrade the server to 19w14a
• Connect with a vanilla client - you may have a crash because the entity ids are wrong

I can confirm this, there are 2 issues here. Fabric should check the registries to see if the client can connect. As well fabric's registry should handle vanilla id changes between versions.

related to #88 kind of