I was trying to show photos with unnamed faces so that I can annotate them. There is an online solution but it requires you to mark all the pictures with no faces at all. Any suggestions?
Edit: After re-reading your question and the provided link I see that my answer is similar but does not entirely solve your problem. From my experience with Photos.app and Lightroom this is probably the best way to approach it if the desired album (i.e. unnamed people) and features don't exist natively.
One option is to create a Smart Album via File > New Smart Album and apply the necessary criteria to filter out existing people.
If you have a small number of people you can choose [Person] | [does not include] and add full or partial names to exclude from the Album.
Note that you may need multiple "conditions" (i.e. lines of criteria) to filter out everyone as you can't use wildcards, comma separated names, etc. This also may not be practical if you have large numbers of people in Photos.
Bulk exclusions (this is what you want)
As a catch all approach you can exclude a set of letters that would cover most names such as a,e,i,o,u as at least one of those letters is present in most names. Make sure to select "All" in the "Match [___] of the following conditions" line. This works because it will do partial matches to including the letter o will match (and exclude) names like John, Bob, Thomas, etc.
If / when you find photos containing people that were not caught by this catch all you can determine which letter(s) are missing and add them to the rule.