This is a followup from FROSCON. So while Kleopatra and GpgOL are tasked with showing the user each time something is non compliant, for signing we don't have the option because while we can encrypt to non-compliant keys in compliance mode, we cannot sign.
This is propblematic if you have for example a compliant workstation but also want to use it to e.g. send out signed release announcements with a different key. Our system just does not allow it.
When discussed we were unsure if this was intentional or an accident in the implementation. maybe a bit related T6643