Systems contribute to more emergent gameplay. They don't kill emergent gameplay.
You're using "emergent gameplay" like it's a goal. If zombies don't dig, the emergent is hiding under a layer of mud like Arnie in Predator. If they do dig, the emergent is hiding "deeper" in ground, or providing routes. Both options produce emergent possibilities; and thus it's not a counterargument for digging having been a mechanic against a player tactic (of covering themselves in mud).
TFP did not make underground bases impossible.
I live in one. There's no screamers, no stragglers, no spawns whatsoever. The digging fails to threaten me to this day. And the perception of it being implemented against players digging remains.
As the arrow slit block completely negated a major aspect of the game I think they were correct to fix that bug.
A force field provides zero actual benefit over a moat, which still works fine. If I'm lazy, I cover my base entrance with a two-deep ditch, you can drive over it just as well as you could the force field, and it blocks zombie pathing just the same. But it was "important enough" to mess up the actual arrow slit function of the arrow slit block.
I do know for a fact that no developer added destroy area mode to the zombie AI in order to stop one particular gameplay choice of players that the devs didn't like.
So the check to do it just happens to happen after being dropped, only when landing within ~10 meters of the target player. Absolutely by coincidink, I'm sure. The results show it's designed against droppers. The way it's implemented also breaks parkour, you can just jump a couple times to get most of the zeds start eating walls... emergent gameplay at its finest.
Now there seems to be a further path-length rule to destroy area, something like "close enough to player horizontally, start breaking stuff if no short path available"; but that's a later addition.
And claiming there is an arms race is supposed to calm people? That's pretty rich.
Lucky I'm not being paid to calm people down then

As in, "it's not my goal, never said it was". If you're hiring, I can make it my goal; but I'd try to do it by sticking to the obvious truths. Such that of course the arms race is happening; if players, QA or devs come up with stupid glitches, the devs want to fix those. Claiming otherwise is just going to frustrate people. They can see it with their very own eyes.
The reality is that they decide how gamebreaking exploiting a block might be or how gamebreaking the AI pathing might be and they make adjustments.
Naturally. The track record of achieving that is a little spotty, as evidenced by the points we're discussing; but I'm sure that's the goal.
One interpretation of this is that they are refining the pathing and AI to make the game more fun. Another interpretation is that they are engaging in an arms race against the players.
A third one, it's both..? You can still juggle zeds just fine, and where you can't, there's usually a failure mode that works worse.. like drawbridges, sometimes zeds just think they're accessible always and run off of them themselves...
What if some players loved the gameplay of juggling the zombies back and forth like it was possible to do in A17 and TFP had listened to them and never made changes so as to not be "anti-player"?
Then no-one would be claiming that TFP has reacted to that particular player design, some players would be happy, and some might complain the game is too easy? Did something change?
What else is there that you know for sure is dev vs player arms racing
I don't know for sure that "I exist", and since your interpretation is the only true one, none. Unless you can see from the above that the difference might be more in the realms of interpretation..