Source?
Nearly 9% of US presidents have been assassinated. Over a much much smaller timeframe. I would bet you would be less likely to have been assassinated as a Tsar than as a President.
That seems substantial, at least compared to Presidents, especially considering the threat of assassination was a serious consideration given all the political intrigue. Whereas AFAIU U.S. Presidents didn't take the threat of assassination seriously until the 20th century, exposing themselves to the public in ways that would be unthinkable to Tsars; or monarchs and, later, autocrats generally for that matter.
On the other hand, likelihood of death per year looks more comparable. For Tsars I get 2.1% (5 / (1917 - 1682)). For Presidents I get 2.3% (3 / (1917 - 1789)) or 1.7% (4 / (2022 - 1789)), depending on the range.
These days autocrats don't invest themselves much in establishing national political legitimacy or institutional legitimacy, focusing almost exclusively on building and grooming a power base where everyone in power has a vested interest in keeping the leader alive. So Putin probably doesn't have much to worry about, excepting perhaps some patriot military General.
EDIT: It's 27% of Tsars and 1.6% likelihood of death per year if starting from 1613, the start of the Romanov Dynasty. (Refreshing my Russian history as I go.) Numbers don't really change if we go back to 1547, Ivan the Terrible and first self-styled Tsar of Russia, as we add another intervening murder. And that's being charitable on the first metric given rapid succession and overlap during the Time of Troubles. https://en.wikipedia.org/wiki/List_of_Russian_monarchs
My guess is both attempts and successes are fewer over time and will approach 0 until some structural change shocks increasingly complacent protections.
