SUMMARY
If I launch some application that would block kwin compositing and then close it (restoring kwin's abilities), next time I restart the computer and it launches a new plasma session (in which said blocker app has not been run), it will launch with kwin effects disabled, and will issue a notification about enabling them with a shortcut. Example app to try: https://github.com/Taiko2k/TauonMusicBox (has an easily installable flatpak).

STEPS TO REPRODUCE
1. Launch some app that would temporarily block kwin compositing.
2. Close said app.
3. Restart the computer and wait until plasma launches.

OBSERVED RESULT
Plasma should launch with kwin compositing enabled.

EXPECTED RESULT
Plasma launches with kwin compositing disabled.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION