They first started a little over 10yrs ago stopping the individual sale of the sensors and making them only available as a set and now it seems they are showing No Longer Available. I replaced mine almost 10yrs ago and only about 10k miles ago and after doing my 6spd conversion lately one of the new ones had already failed but I was able to get it going again by soldering a bad connecton on the circuit board (known issue and it seems even with a newer sensor).
Who knows how long the wait is but it looks like you can at least order the right side from Audi Tradition in Germany. https://shops.audi.com/en_GB/web/tra...erature&page=5