I am aware that notebook provides the switch-page signal, which is essentially the same functionality. NotebookPages not implementing the interface doesn’t lock me out of monitoring page selection, but I wanted someone to sanity check me before I open an issue that the docs have an error.
To be precise: the GtkNotebookPage internal type never implemented GtkSelectionModel. The documentation blurb was wrong when it was introduced in commit c65239155f93eb6fe1d38e203756f706cc415d6d, which was supposedly only a port from the old gtk-doc style docblocks to the modern gi-docgen Markdown docblocks.