What date did the absolute height rule come into effect? And what governing body did it?
Early 2018 to December 2018 to future, depending on how you count:
February and March 2018 saw the German Open and All England play with the new rules. These are very high-level tournaments, but officially it was still an experiment.
At the AGM 2018, Fixed Service height rules were adopted for all tournaments starting December 10th, 2018. The laws were updated to reflect that in December 2018. All of these changes were made by BWF.
Continental Championships (governed by BEC/BAC/BOC etc.) have all adopted the new rules as well. Other Continental Confederation tournaments (International Challenge, International Series, Future Series, various youth tournaments) have largely adopted the fixed service heights, but are not strictly required to yet.
Most national tournaments and federations have not even started to adopt the new service laws. From what I've heard, most national federations plan to introduce fixed service height rules in the season 2020/2021, although some federations will only apply this to their top-level tournaments. I'd expect most players in official federations to play with fixed service height rules starting in 2021/2022 or even 2022/2023. Since the service height devices need to be distributed and solutions have to be found for tournaments without service judges or even without umpires, this can take a while.