I honestly can't see any problem with supporting this feature, even as a player, as long as, like you said, we can easily opt out.
As I pointed, it is a small addition to an already very tested and explored feature, that emailing tool, which itch.io has total control over and the player's data isn't exposed at all (as we can't see the actual emails, just the categories).
So basically it would be just a matter of adding a new category "Free download" or even just "Followers", in which only people that turned on "Follow" are included.
Then it could be debatable if everyone that downloads the game are automatically following and they can opt-out, or if we keep it as is and players can opt-in to that email category by following the project. Either are OK honestly