Even if Github did provide that setting, as a courtesy, someone could clone / fork the code to another repo (if you use any licence that allows it) and not enable that setting.
In a case like this, GitHub itself could set up a bot account that forks all projects as soon as you make the switch. The company in fact would be incentivized to do so.
A repo setting that instructs Github not to use your code for Copilot, it could be a similar option as turning Discussions on / off.
If they really want to win developers over they would even have Copilot scanning disabled by default but that'll never happen.