Strategically placed near every door, window, sewer pipe, and vent the DM put an Antimagic Field device.

I once played with someone who argued that Diplomacy would never be usable on any important person because, since it requires 1 minute of uninterrupted conversation to use, everyone who is important enough would have a jester or aid or someone they'd hire specifically to interrupt every conversation they were involved in every 9 rounds. Absolutely infuriating person to play with. This anecdote is completely unrelated to this post, but your suggestion just made me remember it again, and it irritated me all over again.

A permanent channel of a spell of that high level I would hope would only be available to the most powerful of people.

Also those things would be so expensive they would be worth stealing!

I personally have two mages at entrances that are taking turns ritual casting detect magic. I guess similar impact, but it's not on all entrances.

I'll be damned, I wouldn't have thought wild shape was magic but indeed it is.

Antimagic field is an eighth level spell with one hour concentration duration; an item that has it on 24 hours a day would easily be a legendary item. People underestimate how powerful a spell it is and suggest spamming it everywhere. Having it on every door, window, sewer pipe, and vent would be massive overkill just to spite wildshape.