SegWit fixed transaction malleability by just moving the signature out of the block of data that's being hashed to form the transaction id.
However, Schnorr does not suffer from signature malleability (https://medium.com/digitalassetresearch/schnorr-signatures-the-inevitability-of-privacy-in-bitcoin-b2f45a1f7287):
Non-malleability: ECDSA signatures are inherently malleable, which may enable a third party without access to the private key to alter an existing valid signature and double-spend funds. This issue was formally discussed in BIP62. In comparison, Schnorr signatures are provably non-malleable.
Consequently, is a Schnorr signature still segregated from the data that's being hashed, and if yes, why?