@rednoise0 ,
I just ran an easy quick experiment: I had a random executable which had never been pinned, never been used for Open With or as the file extension handler (and doesn’t do anything special when used to “open” a file, through blah.exe path\to\file.txt; the source code of that executable is tiny, and does not do any registration of Jump Lists or anything; it is a dead-simple program that has nothing to do with file access). I then dragged it to the taskbar to pin it. Then I right-click on file.txt and did Open With, browsed to the exe that I pinned, and chose “just once”. After doing that, I right-clicked on the pin, and file.txt was in its jumplist.
It has nothing to do with “registering” it with the OS. It has nothing to do with whether it’s been there a long time or not.
Further, I had a Win10 virtual machine, which had never had a Notepad++ jumplist before. I made sure Notepad++ was pinned, confirmed there was no list; then right clicked on a textfile, chose Open With and browsed to notepad++.exe (64bit), it opened the file with Notepad++, then when I looked at the jumplist, the textfile was listed.