Is it a fan game (the game itself) or simply posting a link? While the line can sometimes be blurry, what's truly prohibited is selling intellectual property that doesn't belong to you. Fan games, fan films, or even fan covers of songs might be tolerated. It all depends on the leniency of the rights holders.
Viewing post in Sonic Robo Blast 2 Kart ( Fan Page ) comments
I'm well aware of that, and technically I'm not redistributing it. Look closely at the links; they're external links that redirect to the "official" fan game website. I think you don't really know what you're talking about, and if it bothered them, they surely would have told me a long time ago, especially since I posted a comment on their forum telling them I was going to do it if they didn't mind. And that's for the simple reason of introducing the game to the Itch audience, who had a good chance of enjoying it, which they did. And look even closer; you'll see that it's not the only page...