I have noticed today while performing a fresh installation of 21.2.5 that Yakuake does not appear in the following settings and it fails to work as a consequence :
System - System Settings - Shortcuts - Yukuake
System - System Settings - Startup and Shutdown - Autostart - Yukuake
Notably the above settings appear in the live boot from media, but after installation is complete, they are gone.
There was some discussion of this in December 2021 Yakuake does not work in the new version of the system and to be fair I’m not sure if the cause is the same. For me, it was necessary to simply add Yakuake to startup, but that didn’t resolve the missing setting under Shortcuts.