Wouldn't it be better to parse it the way vanilla does and ignore invalid values (evaluate them to always)?

This is just a compensation for buggy plugins using ProtocolLib (which totally doesn't have 200 bug reports).

i think just forward them is better, it guarantees we don't break any behaviour.

If the client is ignoring these values, what about just specialcasing unknown values as null and write them as "null" string? no need for saving as string.
Or maybe we could even attempt to rescue it by checking lowercase in enum as well?

You can send anything the client just ignores it if it cant parse it

You can send anything the client just ignores it if it cant parse it

I think you commeted before I, immediately after posting, changed my comment. I don't see why we need to preserve which ignored value is sent.

I am not sure if every minecraft version handles it exactly the same, and i don't want to lookup the code in any minecraft version bungee supports,
Thats why i just forwarded it, by forwarding we know that nothing will break, and we have the same behaviour as before the team packet changes