openrtb.envelope.domainspec_unsupported
What it means
The 3.0 envelope's domainspec names a domain object model other than AdCOM. The transport layer still validates fully, but the domain objects under item.spec and bid.media are opaque to RTBlint, so they are left unchecked.
Why it matters
In practice the 3.0 ecosystem is AdCOM; a different domainspec is either intentional (a closed ecosystem with its own object model) or a typo that will make partners skip your domain objects. Either way, know that the domain layer went unvalidated.
How to fix it
If you meant AdCOM, set domainspec to "adcom" and domainver accordingly. If you run a custom domain spec on purpose, this warning is informational: validate the domain objects with that spec's own tooling.
Check your payload
Paste a bid request or response into the tester to see whether this code fires against it, or gate on the id in CI with the CLI: the id is stable, wording is not.
Back to the full diagnostic code reference.