I looked in the AIWC patch notes to find out why they were taken out, since the comments in this topic interested me and this is what I found:
From
https://wiki.arcengames.com/index.php?title=AI_War:4.000_Release (using CRTL+F and searching for "turret")
AI's no longer use any form of turrets, and any turrets that the AIs did have are now removed from old savegames (including turret remains). This includes sniper turrets, spider turrets, tractor beams, the works.
Larger-form AI-only replacements for some of the more notably types of turrets will see a resurgence, but they will be easier to see (and thus to strategize against). This will also have a positive performance impact, with fewer AI units doing the job of a larger number of older turrets.
The emphasis is instead on letting the AIs make use of their ship caps with mobile ships that actually pose a natural form of counterattack risk to players (and which are more interesting, anyway, in the hands of the AI).
Part of the reason for the shift away from just huge numbers of fleet ships is our new emphasis on larger centerpieces. The AIs have massive new command stations and guard posts, as well as mobile Guardians that not only defend but launch often-brutal counterattacks. Going along with these are the exciting new AI Eye that emphasizes de-blobbing, the new AI Barracks that lets the AIs store up overflow reinforcements for later use, and the AI Carriers which are the late-game AI equivalent of transports. All of these new things take the place of turrets, which the AI no longer uses at all, and in general they lead to a vastly different feel of game. It's a lot quicker to resolve battles (without making you rushed—just no longer a grind), and in general it makes planets feel more unique and fun before you even get into the various special weapons that have always been a cornerstone of AI War.