how are Mercedes still getting away with'dummy' pit stops ?
https://www.bbc.com/sport/formula1/33461454 this is from 2015 and they did it 2 times in the last race
Formula 1 teams are to be warned they face penalties if they repeat the "dummy" pit stop used by Mercedes during the British Grand Prix.

But teams are to be reminded the practice is against the rules and they could face sanctions in future. Charlie Whiting, the F1 race director for governing body the FIA, said he had "no intention of giving them chances".

It is not clear what punishments could be meted out, but it would be likely to be some form of time penalty during the race.