I have just had the above warning appear on my A5 MY17 2.0 TFSI, with advice that you can continue to drive the car. Luckily, I came across this lengthy thread on another A4 forum which I feel is worth linking, as the fix is simple and the consequences of continuing to drive could be expensive. It’s worth noting that the ABS module can be damaged if you continue to drive with the fault. The malfunction caused by the brake servo sensor seems to be quite common, mainly in the MY17.
Just to re draw everyones attention to this as it is a potentially costly fault.
It seems from searching around various forums this is now a fairly common problem. It manifests itself as a "Brake servo restricted" warning together with a start/stop fault. Unhelpfully it suggests that you can continue to drive the car, however to do so would be extremely unwise.
I have just suffered this problem and was able to replace the brake pressure sensor myself (part no 5Q0906207B) for £79. Part sourced from local Audi dealer. The guy at the desk said they are now selling multiple sensors a week (and it isn't just the A4). It's a quick fix if you search for "brake servo restricted" online. However, if you continue to drive the car you could end up having to replace the entire ABS system for north of 3k. It has happened to people.