I pointed my repo to unstable and upgraded plasma-workspace. The problem
is now solved. I don't see any more zombie kwalletd5 process.
NOTE 1: ksshaskpass was broken earlier. Now, ksshaskpass is able to
retrieve the credentials at login time for adding my private keys to the
ssh agent. I am initializing this by adding "ssh-add" in the startup
programs (which indirectly calls ksshaskpass).
NOTE 2: It is possible that your kde wallet might get closed after being
unlocked upon login. To avoid this, ensure via kde wallet manager
"Configure wallet ..." settings that "Close when last application stops
using it" is not checked.
This package finally solves a long-time problem in Debian :)!