Barr can propose this all he wants
Big Tech will sue, and get a stay from the courts because of the difficulty of implementing this (especially for end to end stuff like iMessage etc.) Then it will go through the courts, and THEY will determine whether it violates the fourth amendment, not Trump toady Barr.
It would take long enough that Barr will be long gone before it is decided, and we will hopefully have much less stupidity at the top of the DoJ.
Biting the hand that feeds IT
