In case that the STS is used only for token mapping, the STS does not need ClaimHandler to lookup claims from a backend.
Example Scenario: Fediz-IDP is used as a RP-IDP only (with no direct user login), but only doing claim mappings.
In this case the STS only needs a relationship with a ClaimMapper, but no ClaimHandler are required.
The following code within org.apache.cxf.sts.operation.TokenIssueOperation however checks if ClaimMapper for requested Claims exists and fails if not.
From my understanding these checkClaimsSupport can be removed completely, because the STS will still fail, if the requested Claims are not available in the end.