timmacman wrote:
Although the ability to find photos with no keywords was available in iPhoto it is not included in the photos.app. I miss this feature a lot and the only decent work around I have found is to create a smart album that has multiple filters of "keyword is not (followed by one of your common keywords)". Its a bit daunting but you can add to it as you find more keywords you need to filter out. I have approximately 50 filters to make this smart album useful.

I do the same thing but it's not easily manageable as, like you, I have so many keywords (and they will sometimes change).
As smart albums are based on rules, include negative conditions ("is not"), and keywords are so fundamental to managing increasingly large libraries, it's baffling that Apple have decided not to include a negative rule for keywords.
Hopefully Sierra will tidy up some of these omissions but I'm not hopeful.