The following files are currently preserved across powerwash:

unencrypted/preserve/attestation.epb
unencrypted/preserve/enrollment_state.epb
unencrypted/preserve/local_tpm_data
unencrypted/preserve/powerwash_count
unencrypted/preserve/update_engine/prefs/rollback-version
home/.shadow/install_attributes.pb

It seems likely that most of this isn't needed anymore now that the TPM is cleared as part of powerwash.