There is a history of bug reports against the usability of the traditional GTK bookmarks feature. The bug reports remain unresolved after many years.
The general problem with GTK bookmarks is the base file that is used. The file is stored in the user’s $HOME as .gtk-bookmarks. In GTK 3, because well, you know, GTK 3 has to be different, that location is moved to $HOME/.config/gtk-3.0/bookmarks. In both GTK 2 and GTk 3 the file is exactly the same.
The file is a text file with two fields for each bookmark. The first field is the actual file path and the second field is the user’s preferred or selected user friendly or human readable description of the file path. For example, in my bookmarks I have /usr/local/bin and /usr/local/sbin. The former I call User Scripts and the latter Admin Scripts.
A common request or complaint against this text container is the inability to add separators so users can improve readability in menus. The underlying code does not support this option.
Adding this support then requires sweat equity in the GTK file picker dialogs.
The GTK file picker dialogs are designed with a presumption of accessing the user’s bookmarks.
Users keep waiting.