I agree that “Minimal soulbound NFTs” is not appropriate, and the idea of SBTs should be divided into two traits: untransferability and non-fungibility, in the context of implementation. That’s why I named EIP-6268 “Untransferability Indicator for EIP-1155”. Actually, I first wanted to rename this EIP (see the first revision).
IMHO “Untransferability Indicator for EIP-721” has clearer meaning.