Let’s be clear—AP1 is capable of lane keep assist. If it can keep you in the lane with AP activated and warn you of lane departure when it’s not, then it can perform the very rudimentary duties of LKA.
True, however the question is. Was it part of the AP1 feature set when AP1 was originally sold? Elon already said in one of his old tweets that AP1 is not going to gain any new features as they concentrate on the AP2/2.5/FSD hardware/software stack.
So, yes while it technically can support it via new software, it's highly unlikely that Tesla will devote any more software engineering cycles to add features to something that has been determined EOL (not the vehicle, but in terms of adding capabilities to MobileEye's solution).
It's not what you want to hear, I know. I had an AP1 Model S P90DL, and I took the plunge and upgraded to an AP2.5 Model S P100D because I wanted the newer tech, and something that is expanding it's feature set and upgradable to the FSD computer later at the end of Q4.
It sounds like if you want the newer tech, an upgrade may be in your future.