Autopilot 2018.26
While my Model X, Pensive, was in the service center, the technicians updated the software in the vehicle to 2018.26. While Tesla broke with tradition by adding information about new features to the Release Notes, they were consistently laconic when it came to changes to its AutoPilot behaviour.
My initial observation: It no longer tries to kill you, as often.
Look Tesla! Still no lines!
The Autopilot system is more likely to go to "Take over immediately" warnings than previous iterations; you could argue either that this is a regression—it is no longer as confident as it was previously—or more specifically that it is a regression to the norm, as it more closely matches the behaviour of other solutions which aim to be more conservative. When I took it down the same test road I've used to compare earlier versions, the car made it slightly farther (around the first bend) before warning about taking over control immediately as it couldn't handle the switchback.
When the road first loses the lines, it locks onto a line caused by filling a crack in the road, and treats that as the lane marker, which works wonderfully initially (it' centered), but breaks down as it moves to a 20/80 split in the road further-in. If a car slows enough (e.g. to make a turn), the car will slow with it, but upon the car completing the turn, it will immediately launch into take over immediately.
Take Over Immediately
I've mentioned the take over immediately warning a couple times already. It is much more unpredictable when the warning—or even the hand requests—are presented in this version. The request to demonstrate your hands were on the wheel might sometimes appear twice with-in 300ft, while "Take over immediately" even appeared while stopped at a light (with my foot on the break).