(Task originally authored by alexk, see https://dev.gnupg.org/T7089#188791 for reason to recreate it)
There is a long available feature to "disable" keys in GnuPG. This deactivates the local certificate for regular use. Kleopatra should respect this as well:
In certificate view show "disabled" (de: "inaktiv") in the "User-IDs" column.
In certificate view add "disable" or "enable" in the context menu.
In "Details" view add "disabled" in bold next to "Valid until" in case it is disabled.
When searching for certificates in "Encrypt for" fields, don't show disabled keys in the drop-down menu.
A (context) menu entry to "disable certificate" (or "enable certificate").
And either:
make a filter showing "disabled certificates".
don't show disabled certificates in any of the others filters.
or
make a toggle to show/hide "disabled certificates" everywhere
Info: The "disabled" status is stored in the trust-DB. It can be changed with "gpg -edit-key" and "disable".