ssa.gov supports id.me and login.gov[0] so I don't see why IRS couldn't rely on login.gov, which also does identity verification[1].

0: https://secure.ssa.gov/RIL/ (gonna have to wait till tomorrow morning to see it, as it has hours of operation for some reason)

1: https://www.login.gov/help/verify-your-identity/how-to-verif...