I don't think it's hidden, because regardless of it's setting, it is set at runtime?

For me, privacy.clearOnShutdown_v2.downloads gets not set from the dialog clearOnShutdown dialog on about:preference. Neither true nor false. Tried in a new profile.

The sync pref for this pref is not hidden.

https://searchfox.org/mozilla-central/search?q=clearOnShutdown_v2.downloads&path=&case=false&regexp=false

I need to think more about what triggers this. But sure, it's a "hidden" pref (which I knew cuz it has a recycle bin) - but we've had those before not labelled when they are added by runtimes (IIRC)

edit: IDK if it even does anything by the looks of it

toggling settings (in the UI) in nightly doesn't create the pref

So in https://phabricator.services.mozilla.com/D194136 scroll down to browser/base/content/sanitize_v2.xhtml line 234 it was added as the pref behind a checkbox called item-download-history at the end under site settings

but if you look at https://searchfox.org/mozilla-central/source/browser/base/content/sanitize_v2.xhtml it's gone

edit: https://bugzilla.mozilla.org/show_bug.cgi?id=1888466#c7 indicates they considered a separate check item

closing: I'll add a hidden tag to the next version PR because it's clearly hidden, and I literally don't care if the pref is used or not - i'll leave it in out of an abundance of caution