This document proposes an overview of Digital Identities on the Web and an analysis through different use cases of the systemic impact on both the market side and the human side, as well as the role that Web standardization may play in managing that impact
a good starting point might be to separate whose identifiers we are talking about:
for issuers and verifiers, https url, x509 are common (and DIDs)
for holders, some implementations use cnf claim, without explicit user identifier (which is sufficient for batch issued credentials) (and DIDs) (and URLs if holder is a legal entity, but I think this is still WIP, so can be added later)
a good starting point might be to separate whose identifiers we are talking about:
https://openid.github.io/OpenID4VP/openid-4-verifiable-presentations-wg-draft.html#section-5.7 is one of the relevant sections just to showcase different options