I just noticed they have disappeared...but when, and why?

I think, they disappeared a few weeks ago?
Why? Ask a blue.

But hey: after a run of two years and (almost) three months, the "improving phase" is probably over and it was time for a change.../s
https://www.gog.com/forum/general/gog_2022_update_6_opencritic_reviews_on_our_gamecards_51e5b

I hope it's some temporary hiccup and not an intentional permanent change. I liked having those there. I'm not saying they are the end-all, be-all decision making factor by any means, but it's convenient to be able to get stuff like that and HowLongToBeat all in a quick glance at the game's page.