I just realized that this search feature would also fix this other request for creating a list with other people observations.
Indeed, it would be enough to put a keyword in a comment in an observation. Then, searching for this keyword in all my comments creates de facto a list of observations (under my exclusive control, which might be a limitation).
Some iNaturalist users prevent others from adding their observations to a Traditional Project to which they have not subscribed. But they can’t prevent others from writing a comment in their observations. At least, all observations are open to comments. We got it!