It's supposedly only applicable to the "super wealthy".
That being said, once that door is open, who knows how wide it may swing in years to come. We already see instances of "unrealized gains" in the taxation world. Anyone who owns properties knows that when their annual property taxes are raised, it's from a a theoretical rise in value, even though the property owner has not realized a tangible cent of that rise in valuation. It's just a number on paper.
The people who pretend this is a nothingburger are either partisan hacks or historically unaware.