Hello @Harold Harris - Thank you for your post!
The support statement for workgroup and untrusted domain is same for both DPM and MABS. For MABS also we point to DPM documentation.
It is documented in the same article here - MABS (Azure Backup Server) V3 UR1 protection matrix - Azure Backup | Microsoft Learn. For some reason the link is redirecting to incorrect document. Will get that fixed.
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