There was more at that link… which was similar to the work around in the bug report (comment 23).
Putting them together:
.config/kwinrc
[ModifierOnlyShortcuts]
Meta=org.kde.plasmashell,/PlasmaShell,org.kde.PlasmaShell,activateLauncherMenuqdbus org.kde.KWin /KWin reconfigure
If you have qdbusviewer installed you can follow the path to the method:
org.kde.plasmashell > PlasmaShell > org.kde.PlasmaShell > activateLauncherMenu
If you double click the method it will open the launcher. This is just an educational or verification step.
I do not have this problem, but KDE isn’t my main machine anymore. I looked in my kwinrc and didn’t have a ModifierOnlyShortcuts section, but I added it anyway along with the next line and restarted kwin. The Meta key still worked, and I didn’t appear to break anything I am using the Application Menu, but changed it to the alternative, Application Launcher and the Meta key worked there too. I set Meta= nothing, to confirm I could unassign it and that worked too. Absent any other fix, this is one option to potentially try.