Really nice find! Your method is working in Photos 7 in Monterey as well. It would have helped, if Apple had mentioned it in the "What is new" section of the Photos User Guide to give us a hint, where the feature has been moved to.
I now found it 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.
- In the Photos app on your Mac, open a video clip, and drag the playhead to the frame you want to export.
- Choose File > Export > Export Frame to Pictures.
- A TIFF file named “Frame [date]” appears in the Pictures folder.
I can no longer test on Big Sur. Could you please just clarify, if you have been testing on Monterey or Big Sur? It would help the Big Sur users to know, if this feature has been available in Photos 6 on Big Sur all along and we never found it.