Given Git is distributed (or most source control as a matter of fact today is) - even if you stopped pushing your code to Github, does it stop Copilot from pulling code from sources like gitlab.gnome.org, kernel.org, gitlab.kde.org etc?
I think underlying discussion should be about licensing, not about website to which you are pushing open source code to. Because that can be easily worked around.
There is a hidden problem with licensing here. Developers are giving Github the permission to use the code with a different license [1]. The clause sounds broad enough for them to justify training copilot with it. This allows them to disregard the license with which the project is published. The developers don't have the protection of a FOSS license anymore when you host there.
I think underlying discussion should be about licensing, not about website to which you are pushing open source code to. Because that can be easily worked around.