@TheEdVerse in 1371DR what would it take to gate JUST the Yawning Portal & those in it to the Demi-Plane of Shadow? Temporarily of course.
Asking for a friend.
Ahem, to amplify just a little: to do that, you'd have to breach a hole in, or break outright, the wards of Waterdeep.
Picture a pancake (the surface city) balanced on a grille of three widely-spaced bars (the wards), atop a pit (Undermountain).
To pluck a dime-shaped piece out..
...of the pancake, using tweezers or tongs, and pull it away (removing the Yawning Portal), is going to tug on the pancake.
With a high chance of it falling between the bars of the grille, down into the pit.
Which is why the wards are designed to resist any such endeavour.