How to extract a still frame from a video in Photos 7 on macOS 12 Monterey

User chewch discovered that the extraction of still frames has been moved to the "File > Export" dialog: https://discussions.apple.com/thread/253344584?page=1

The method has been described in the user guide in the section Export photos, videos, and slideshows from Photos on Mac - Apple Support of the Photos user guide, the paragraph "Export a video frame as a photo":

You can review a video clip to identify an outstanding frame and export it as a photo into the Pictures folder.

1. 1. In the Photos app  on your Mac, open a video clip, and drag the playhead to the frame you want to export.
   2. Choose File > Export > Export Frame to Pictures.
   3. A TIFF file named “Frame [date]” appears in the Pictures folder.