I’m probably a little oversensitive about this considering the fact that this change is screwing over people like me, but eliminating this deduction was in no way “closing a tax loophole.” Eliminating or reducing the deduction means that taxpayers are being taxed on the same income twice.
Until 2018, the SALT deduction has been around as long as there has been a federal income tax (i.e. since 1913).
In fact, there was a time when federal tax rates were so high (a top rate as high as 94% in WWII, and still as high as 70% up through the 1970s), that you could easily have had a situation were the combined tax rate of federal, state, and local taxes could exceed 100%, which makes absolutely no sense. In this situation, it would actually penalize you to make over a certain income.
Which is why is makes sense to be able to deduct the money paid for state and local taxes. You never see the money, and there is nothing you can do to eliminate it (unlike, for example, the deduction for mortgage interest). Why does it make sense to include this in your federal income (or conversely, why shouldn’t you be able to deduct this from your income for federal taxes, like you’ve been able to do for the last 100+ years)?