If I understand you correctly, the issue arises when you want to trigger a line about the event in a diagram or something. In general, of course, just leaving the date empty is enough to mean "Unknown". But you still need the Event in the first place.
I just tried to use the All tab to add a Death event, but omitted any date or place. On my diagram, this triggered a line containing "d." only. Which seems adequate to me.
It even seems to work if I add a Death event on the Facts tab of the Individual's property box.
Adding it on the Main tab of the Individual's property box seems more difficult as spaces in the event details don't seem to be recognized as input - reasonably enough. In that case, you might consider putting a question mark in the place name. Naughty, of course, from a pedant's view.
In all those cases, the narrative reports say "She died".