Since Novembre 2016, Google has “improved” the Google Photos app, but they simply removed all filters for something that is not equal… I’ve sent a feedback through the Google Photos contact form (https://photos.google.com/ > left menu > give feedback).
The workaround is to download an old version of Google Photos and install to a simulator or on your phone. You can find all APK versions on the APKMirror website.
If you don’t know which version is needed for your phone, check on Google your phone architecture version and dpi. For example, for a Nexus 5X, it’s arm64 – dpi 420 (default), so you need to download the 540717 (October 31, 2016) version which corresponds to: arm64, Android 4.1+, 400-480dpi
Google Forum – Is there way to use the previous filter?