I'm looking for the same thing, did you ever figure out a way? In trying to fix this, I have disabled update notifications everywhere in UnRaid that I can find, but unassigned devices doesn't seem to respect those settings.
I let the dust settle after a storm of updates so others can beta test before I accept them, so the repeated notifications are annoying (I love the plugin, I just don't need to be informed that there were minor tweaks and corrections multiple times a week!)
I see the replies saying that update notification banners are an UnRaid thing so they can't be disabled, but other plugins don't seem to do it on the main UnRaid page (that I have noticed....)