Yes, there is an account setting checkbox labeled "Allow GitHub to use my code snippets for product improvements" - https://github.com/settings/copilot
I'm a bit confused how this will work in practice, because I don't see how they can retrain their entire model every time someone toggles that setting.
You can't access that page or toggle that checkbox unless you pay for Copilot first. This might be the most literal case I've ever seen of "if you're not paying, you're the product"
Were you part of the Copilot demo/preview? It appears that doing so leaves your github account in some sort of mostly enrolled state for Copilot that lets you tweak these settings compared to an account that wasn't part of the preview that just takes you to a "start your free trial" page.
'Substantial portion' is not interpreted by you, it is interpreted by courts and juries. How it is interpreted depends on jurisdiction and more[0], the commonly accepted 'safe bet' is any single line of the software.
I'm a bit confused how this will work in practice, because I don't see how they can retrain their entire model every time someone toggles that setting.