just based on a cursory search for not_id in the iNat repo in Github, it looks like maybe 750 obs. i did a quick test to exclude 600 obs via not_id, and it worked as expected.
if you’re doing things at a user level, i think that would cover 99.9% of users, since i wouldn’t expect most people to have more than 750 observations of dead stuff.
maybe by the time there are people who have accumulated that many observations, the API will have been updated so that the exclusion filters parameters for annotations work better.