just to clarify, the parameter has never been implemented. (if it was implemented though, it would definitely make what you’re trying to do much easier.)
probably the most straightforward thing would be to just export the observations from the project, including the observation field as one of the export fields. (the observation field will become an export field option once you add it to any observation.) then just filter the results to look for observations without that field populated.
exporting observations seems to be a bit slow nowadays though. if you know someone who can code a bit, you might be able to leverage the iNat API to get your project observations and then drop the ones with the observation field added. that might get the observations you’re interested in faster, though the API route is probably not a great solution if your project grows to more than 10000 observations.