SUMMARY
Sorry if this issue is already reported but I searched the most relevant keywords I could think and was quite surprised I couldn't find it. The problem is: there is no user-friendly way for deleting a widget that is not being rendered on screen (but it would be nicer if it was recoverable).
In my opinion this is a major problem on user trust, I made the simple mistake of trying to move a widget from a panel on one screen to another, a simple and unassuming operation, and when my widget just vanished in thin air and I went to create it again (sighs I need to reconfigure it all over) I noticed the widget explorer would still list it as existing... 

And I was not expecting what went ahead of me, first I thought I would just right-click and delete it somehow but no. Okay, tried google... okay what is plasma-org.kde.plasma.desktop-appletsrc... what is Containments??? Applet 199? 27?
You will find, as I did, a lot of user with the same problem and the same answer:
https://forum.manjaro.org/t/kde-widget-how-to-remove/78193
https://discuss.kde.org/t/how-to-delete-hidden-desktop-widgets/13898
https://unix.stackexchange.com/questions/620699/remove-widgets-which-are-off-screen
https://forums.linuxmint.com/viewtopic.php?t=267864

"well, just edit your plasma-org.kde.plasma.desktop-appletsrc"

As it was not some esoteric inconceivable mumble of machine uttered poetry. This is no place for humans.

Well, not considering me much of a human I ventured forth... just to mess my entire desktop, do your backups, lesson learned.

But I really think such a simple task should not come with such dire adventures. This is an really normal and common user desire and with consequences that can make people really reconsider what they are doing with their lives. Well this is my story, and I dare say many silent others too, hope it reach a caring heart.
 
STEPS TO REPRODUCE
1. Disappear with a widget
2. Try to remove it
3. Get lost on the shadow realm

OBSERVED RESULT
This is not a task for mere mortals.

EXPECTED RESULT
Right click on widget at the widget explorer > list widgets > place it on screen again (or delete it if getting it pack from shadow realm is unfeasible)

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.6.46-1-MANJARO (64-bit)
KDE Plasma Version: 6.0.5
KDE Frameworks Version: 6.5.0
Qt Version: 6.7.2