I made a suggestion for another notebook which got merged shortly after. After the suggestion was merged I deleted the forked notebook as I no longer needed it.
My activity log shows no trace of any of these actions. 
Edit: I would also consider the act of deleting a notebook an activity that’s worth being logged. 
If there are no plans to extend the time span for the log past 30 days, then deletions could perhaps be displayed for notebooks that are still in the trash. Once someone decides to empty their trash manually they’ll probably (and happily) accept that those notebooks also disappear from the activity log.
(With the caveat that there are a great many improvements we’d like to make to the activity log, including showing notebook deletions in it) — activity is now displayed for notebooks currently sitting in your Trash.
Thank you! Would it be possible to add a visual distinction for trashed notebooks, though?