Hi all, just to add my own experience with this - i got the same warnings this morning (brake servo restricted + start/stop fault). At first i decided to ignore as it wasn't in yellow or red. But then i got on the internet spent couple of hours reading other peoples' experiences and i got scared (didn't want to pay £3k for new ABS system). I called my local workshop, who said they didn't do such things. Then i called Audi to book an appointment with them, but they don't work saturdays. I tried to book online, the first available slot was next saturday. Then i decided i'm going to replace it myself, looked quite easy on youtube, part available on ebay...
And then (after reading the last posts here) i remembered i have audi roadside assistance cover🤦♀️ I called them straight away, they came in about 1h, the guy was an audi guy and he was amazing. He knew about this fault of course, he said a lot of cars have it and he was really glad i called them straight away on the first day the message appeared. He said he wouldn't recommend driving the car with this fault as it only take certain amount of presses on the brake till the ABS system breaks. Luckily he had exactly the same part with him (apparently there are two types - A and B but he had one of each with him) , he replaced it in no time and diagnostics came out clear, message disappeared...sorted. All covered i didn't pay anything extra.
He also said that even if he had not had the part with him, he would be back here on Monday with it. So if you do have roadside assistance from Audi, just call them. Don't wait, it sounds like it can get really pricey. Also, if you call audi assistance, you're actually calling the yellow AA and they can dispatch any car - it can be the regular AA or Audi guys. Just insist that you want the Audi guys, even if you have to wait a bit longer. The regular AA won't have the parts and probably won't even know about this issue.