The betting platforms can't open markets on Trump's death for obvious reasons, but Kalshi just so happened to open up a "Trump out as President" market on Saturday
Polymarket opened their own on September 1.
Unlike Polymarket, on Kalshi these markets do not resolve to "yes" if the person in question dies (they settle at the last traded price). I suspect many bettors don't read the fine print, though.
Donald Trump Jr. is an adviser to both Kalshi and Polymarket, btw. (This is deeply weird for all of the many reasons having Don Jr. as an adviser is deeply weird, and also because the companies are the two major competitors in the prediction markets space)