The devil is in the details. We, the users, want our bookmarks to be remembered. But what is bookmark?
Is it line number?
Is it line text?
What should happen when file is modified externally?
If I put a bookmark on line 100 and adds 10 lines at the beginning of file, bookmark will jump to 110. Now, when I close the file without saving it, what should be remembered? There could has been 1000 different edits that added and removed random lines.
What if the bookmark was placed on a line that was never saved?
For IDEs that are workspace oriented things are (a bit) easier because bookmarks can be saved as part of the workspace meta files. Notepad++ is not workspace oriented.
Having bookmarks saved as part of the session is a reasonable compromise. I don’t think that core Notepad++ should go beyond that. Find someone to implement Long-Term-Bbookmark-Storage plugin.