SUMMARY
When appmenu plasmoid is added exclusively to an unplugged display, neither menu is accessible nor appmenu plasmoid could be deleted via GUI. Editing ~/.config/plasma-org.kde.plasma.desktop-appletsrc is the only option.
This could be crucial for some portable devices like laptops.

STEPS TO REPRODUCE
1. Plug in the second display
2. Add appmenu plasmoid e.g. as "Application menu bar" panel to both primary and secondary displays
3. Unplug the display
4. (Optional) reboot/restart DE/computer/forger about the display entirely
5. Delete any existing appmenu plasmoids available from the primary display.
6. Restart Plasma session
7. Launch any application known to work with appmenu

OBSERVED RESULT
Menu is not accessible at all.

EXPECTED RESULT
Ideally appmenu is disabled since plasmoids are not loaded.
Possibly some explicit setting is avaliable to disable appmenu plasmoids (like pre-5.12) OR to delete all existing instances of an appmenu plasmoid.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux (latest 2019.06.13), presumably distro-unrelated
KDE Plasma Version: KDE 5.15.*/5.16 from extra