CMK 2.1.0p4 REST API - Bake and sign agents requires key (again)

CMK version: 2.1.0p4

Error description:
The changes made to the REST API in 2.1.0p4 prevent the use of a fingerprint for the “Bake & Sign” operation. Instead a key_id is requested.
But as stated in this werk, there is no real way of getting the key value of the Agent signature key (only way I found was analyzing the WATO download link for the signature key).
Is this an intentional change? If so, is there a way (preferable via REST API) to get the key_id?