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I agree, that's how it should work. GitHub as a third party probably had better TOS than most, and would face more uproar if they changed it. But they are not a social media site.

But for regular social media posting, Facebook, Twitter, etc, there is no fighting against their TOS other than abstinence. You can object in writing with one of those footers, but nobody in charge is going to read it or honor it. It will be shoveled into the AI along with everything else. Your only recourse will be an expensive legal fight, and it would be difficult to prove any particular post was in the AI or not. It's unproven legal ground to say giving a post to an AI for training even qualifies as a "copy" under copyright, or what notification qualifies to exempt your content.