Apparently, the distortion on my screen is worse than I thought. Thank you for the correction.
Game Jolt, as in the fan community site. That site looks like it’s entirely for kids. (I’m not one). Anyhow, it was easy to find this without registering.
The Game pages are categorized as All Ages, Teen Content, and Mature Content.
There were six entries in the Games section with Mature ratings. Some were actually for games. None look like they were made for adults. One had a Maturity with no sublabels on it.
The maturity ratings aren’t really age ratings on an adult-centered site as much as content categories self-selected by the page author.
I really can’t imagine this going far on Itch.io.
But FWIW, I partially agree with you. That rating screen is less intimating than the Sensitive Content screen for “Adult NSFW” here on Itch.io. Having the option to select from a list that auto-formats could be easier than us all deciding what and how to put in a content note on the game page.