The issue of transaction malleability was solved via
BIP66. Further, as you note, SW parent transactions are not maileabile.
BIP-66 is only one case of malleability fix involving the DER encoding used in ECDSA signatures. There are more cases most of which are outlined in
BIP-62. The rest are considered non-standard transactions not invalid in legacy transactions. However they are ineffective in SegWit transactions (signature doesn't affect txid).