Is your feature request related to a problem? Please describe.

Describe the solution you'd like
It would be nice to have a connection predicate that matches a block tag instead of having to list all blocks using match_block. This would also allow other mods to add more variants of the block without having to change the original blocks connections.

Datapack stuff like tags is handled server-side and synced to the client, hence it is not really ideal for use in resource packs as the pack may randomly break when playing on a server.

Can it still be added with the caveat that tags can break? I use this in a mod so I'm guaranteed to have the tags

The problem I'm having is that in the base mod I have list of different colored blocks that all need to connect
https://github.com/JDKDigital/productivemetalworks/blob/dev-1.21.1/src/generated/resources/fusion-overrides/assets/productivemetalworks/models/block/black_foundry_tank.json
and then in the addon mod I want to add more colored blocks that also connect to the base blocks but when I try to overwrite the model files to add the new list of blocks it doesn't work
https://github.com/JDKDigital/dyenamicsandfriends/blob/dev-1.21.1/src/main/resources/fusion-overrides/assets/productivemetalworks/models/block/black_foundry_tank.json

Ah yeah for a mod that is probably fine as you can include datapack stuff with the mod. Might be a while before I get to adding this though.

in the addon mod I want to add more colored blocks that also connect to the base blocks but when I try to overwrite the model files to add the new list of blocks it doesn't work

That sounds like the resource pack for your addon is ordered before the one from the main mod. In your neoforge.mods.toml for the addon you can specify ordering = "AFTER" for the dependency on your main mod and it should put your addon's resource pack after your main mod's resource pack.

After thinking more about the tag stuff, it really doesn't work well for model behaviour to depend on tags. Tags are only available when in a world, whilst models are created when the game is launched and can be rendered at any time, even without a world context.

Depending on tags would also make model behaviour inconsistent when reloading datapacks. Already placed models would remain as is and only update when the chunk mesh they are in is rebuild. This would make it rather confusing when trying to work on a resource/data pack.