I personally define as dead MNOs the Dash collateral addresses who used to have 1000 dash, but they sold all their Dash and now have 0 dash in their address.
The ones who still have 1000 dash but they do not vote, or the ones who still have 1000 dash but they do not have a masternode server, they are not defined as dead by me. I define them as asleep or as inactive.
In https://mnowatch.org/ we ask queries.
Would you like a query like the one you said to be added there?
If yes then please specify exactly what you want, and I will try to code it for you.
Then call it "sudden death". Many people don't get to know when it's going to happen, they just wake up in the morning thinking it's going to be a normal day. Not everyone gets to grow old, or wise enough to make plans.
Someone selling all their dash prior to death seems unlikely to me, and it's more ambiguous where the money went after it was sold. Conversely, if someone has an inactive balance between 1000 - 1999 dash, we can reasonably ask why they would not run a masternode and not put that money to work elsewhere. And given most hosting plans are monthly or annual, anything stagnant after one year would seem a likely candidate. In short, if we know how many people could run a masternode but they don't, I think we can safely say that person is dead or they have lost their keys.