Triangle "unequal traction" warning lights with slight turns on dry pavement, gets more sensitive as you drive. Eventually sets "4matic" warning light and trouble code 11 flashes (steering angle sensor fault). Is it sensor, slip rings or other?

I haven't worked on a 4 matic but, transferring knowledge of how systems work have a look at this.

Is this a new problem or has it gotten worse over time? How long have you had the car?

Are all the tires the same diameter / inflated properly? Matching the size printed on the side wall isn't enough as rolling diameter can vary between manufacturers or due to tire wear. Measure from a fixed point on the wheel or hub to the ground. How much is too much? It depends on the error code strategy.

I've seen erratic ABS sensors ( Ford Explorer ) cause AWD error codes. The area under the front wheel speed sensors would rust, expand and lift the sensor. Mix in a little bit of wheel bearing wear / noise / play and the output signal would become marginal causing the AWD to engage or ABS to think the wheel was locking under braking. Even though you might not have ABS issues, wheel speed sensors might be a issue as the systems are likely separate boxes.

Did the steering angle sensor error give any specifics? ( Signal high / low / erratic / implausible? ) Depending on the error strategy, based on wheel speed variations the computer might think the steering wheel is turned but it really isn't. In this case it says " Hey, the car is turning based on wheel speeds but the SAS signal says it is straight ahead therefor the SAS is incorrect."

Another possibility is that the SAS isn't centered causing the error.

I believe the steering angle sensor is in the steering wheel. I had the triangle illuminating on mild bends at 60 kmh, and exchanging the steering wheel fixed that problem.