If a keypair does have at least one encryption subkey, but none of them is valid, it should be highlighted in the certificate list that something is not as it should.
Edited 2025-05-06:
This should be done by:
- adding as icon a crossed out lock
- in VSD versions use an orange background
- in the details view, we show the info "no usable encryption subkey." in the "Status:" line after the current information (which is e.g. certified)