You could be a bit more specific in what you mean by the “Overlord Clause”.
Are you talking about this part?
[Publishers grant] To the Company, a worldwide, non-exclusive, royalty-free, sublicensable and transferable license to use, reproduce, distribute, prepare derivative works of, display, and perform the content in connection with the Service, including without limitation for promoting, redistributing in any and all media formats. If you choose to remove your content from the Service, this license shall terminate within a commercially reasonable time after you remove your content from the Service.
If so, this is not illegal, because you agreed to this when registering an account. That is the purpose of a Terms of Service. And it’s to Itch only, not to everyone.
From what I see, Itch respects DCMA claims; it just doesn’t take initiative.