Yes, you can see the tags, and the games in them, but the game pages themselves will show an age check if you click through. Arguably the mouse-hover popups leak information, but that's another story.
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I think there are requirements to not show any adult content in thumbnails though, so why would that matter? I strongly dislike opt-in because it limits visibility to the adult audience as well, many of which do not log in when incognito, etc. Thumbnails are the advertisement, blurring them is making a judgment call on what content should be advertised. The age gate when you click should be plenty.