The option copy_on_windows suggests the option is either Windows only or behaves differently for Windows.
As far as I can tell all the option does is enable symlink traversal. Symlink traversal can make sense on other OSes as well:

• The symlink might contain a relative path, in which case ignorantly copying it will break stuff.
• I might be copying to a different drive (for example copying something to an USB drive), in which case I'd not be happily surprised that all I got is a pointer to the location of the data on the original machine.
• I might be preparing a docker context which does not support symlinks.

I think the option should be called follow_symlinks. I propose changing the name of the argument and updating the documentation. The old option can still be supported for BC but should be marked deprecated and be removed in a future BC break.