This is going to sound like a long winded explanation.
None of the defensive builders have been removed, BUT the economic requirements for the defensive builders have been increased.
The reason for this was due to the AI destroying its economy on every game when it wasn't using large aix multipliers which resulted in a human player being in one phase of the game and the AI being two phases behind.
The other impacting reason is because the economic upgrade triggers have NOT been touched, so the AI has spare economy but it is throwing that economy at mass extractor upgrades rather than balancing its spending. This was supposed to be rectified in a structure manager feature that would better control mass extractor upgrades but this wasn't ready in time for the 3rd iteration so the AI's economy is still out of whack.
What will happen in the future is that we will get the structure manager sorted which will allow AI to better balance its economic distribution, there is a feature for defensive structure placement that is also due to be implemented. Then we will rewrite how the AI builds defenses in general and implement a better focused defensive logic for the turtle AI personality with the other AI having a more efficient reactive defense build logic.
So post the replay link and I'll take a look and see if the reason it isn't doing defenses is purely the above or if there is some other issue that can be fixed.