This is interesting. I wonder why it's not visible by default.

Maybe it is/was an A/B test, to see if it hurts ad revenue (it probably does).

The option appeared randomly for me on a search, and I took immediately note of the udm number. :)

In addition to the others mentioned on the sibling comment (I cannot reply to it?), udm=28 is shopping, 36 is books, 37 is "products", 44 is "visual matches", 48 is "exact matches", 50 is "AI Mode" but no tab appears, 51 is homework and I stopped at 80 because the page kept removing that part of the url all the way up to that point.

For some reason the reply button won’t pop up right away but you can click on the post’s timestamp and reply there

Ran through the lower numbers that hit something interesting:

8 = jobs (but doesn't return any results) 15 = attractions (but doesn't return any results)