If this help anyone, here's my experience.
I've just had the brake servo restricted message on my 2016 A4. I then drove it 20 miles to work the next day without the message. I started looking on here and from what I read I rang an independent AUDI specialist. They told me not to drive the car and have it recovered as the ABS could get corrupted and seize, which would result in a £3500 bill for new ABS unit. Car was towed under my breakdown cover no questions asked. £234 later and the car seems fine today. Charged 2 hours labour to diagnose and fit the sensor, £12 ODIS charge, which seems excessive. But, its not £3500.
When I picked it up the guy said the the error count had reached 2, and when it reaches 7 then the ABS is corrupted. Id only driven it 30 miles in total and had 1 on screen dash message.
AUDI definitely should be ashamed!
