Doesn't nostr use BIP340 keys? A private key that has a public key with odd parity is not invalid; you just use the corresponding even parity point in encoding. It's effectively using -x * G for specific algorithms (see the "Default Signing" algorithm in bip340), but it's not that x is an invalid private key.
Honestly the x-only encoding was a bad choice, it has led to a lot of confusion and complexity.