Wanted to create a ticket for this as well, bumping instead, deleted bookmarks should not be kept without the user knowing it is kept.
I often delete bookmarks I don't consider relevant anymore, but knowing they're being backed up in Browser/TorBrowser/Data/Browser/profile.default/bookmarkbackups forces me to check that folder every few days and purge it.
EDIT: Can be disabled by setting browser.bookmarks.max_backups to 0 in about:config
Binding this pref change to the option to disable Private Browsing Mode in Tor Browser (Don't record browsing history or website data (enabled Private Browsing Mode)) seems the way to go here. Patches are welcome!
I'm not convinced that disabling bookmark backups is a good idea, but it might make sense to reduce the number of backups that are kept by default. It is entirely possible that the SQLite database can get corrupted, and if you're not manually backing up your bookmarks, you'd lose of all of them.
Tying this to private browsing mode doesn't make any sense to me, because bookmarks are kept regardless of this setting.
I can see how up to 15 (default) days of daily bookmark backups could be an issue: assuming your device was compromised: i.e old bookmarks recently deleted would have a persistent disk write for the duration, but what if a user needs to restore them. I also think if your device was compromised, you would have more serious issues than this.
That said, I'm not totally against it: but maybe changing the default value to 2 or 3 might be a better compromise: at least a value of 1 gives a user a chance to instantly retrieve something
I haven't tested anything: so I'm not sure if changing the value removes old ones on a daily basis until it catches up, or if they persist