This is similar to the intent of #11617, making element creation explicit. I think #9521 will prevent most cases of abuse, but certainly if a dev takes arbitrary user input and passes it to
$() there is still a chance it can happen.
In practical terms you won't be able to use the option you describe with any code but your own, it's similar to stopping a nosebleed by putting a tourniquet around your neck. The entire jQuery ecosystem knows that
$("<html stuff>") creates DOM elements.
I'd like to start with an educational process. Nearly all the problems I've seen are due to people taking querystring parameters and using them as selectors, which never was a good idea in the first place. Adding a flag that is only in the latest version and nobody could use anyway is not the right starting point IMO.