Salts are used for things like password storage where multiple users may be using the same password. By using a salt it obfuscates to an observer of the database which users are using the same password because of different salts. The only motivation for using a salt on Taproot scripts would be if users were using the same Taproot scripts. But at the very least users' public keys will be different (unless they are using the same private key) and so no Taproot scripts will be the same unless they are controlled by the same user.
To state the obvious users shouldn't be using the same password (terrible password hygiene) for non-Bitcoin services but it is even more serious for Bitcoin. If users don't generate a Bitcoin private key with sufficient entropy their Bitcoin will likely be stolen.
For the observation, it is possible to guess the unused script from the hash.
Guessing the preimage of a secure hash function is infeasible. It can only be done through brute force (trying lots of possible preimages) but there are so many possible preimages it is like looking for a needle in a haystack.