I have just had my 2017 A4 serviced at Finchley Road. Two day s later I have had the Brake Servo: restricted message.
I have driven about 25 miles since then in getting back home.
I've found this thread thank goodness and have just been to Audi Oxford to pick up the replacement pressure sensor for £80. I haven't driven the car since and hope it isn't already too late for the ABS, and am going to change it tomorrow.
The guy at Audi Oxford tells me they are selling a lot of these items, just goes to show how common the problem is. Pretty scandalous.
Fingers crossed I'm not too late, but what a great thread!