In the recent years, one of the most frequently asked questions from artists, labels and managers has been the playback levels in the various streaming services. As this has been a somewhat moving target since the beginning, it's hard to give an exact answer what the future will bring in terms of a common ground, but I'll try to shed some light on the current state on the most popular platform; Spotify and my overall philosophy about loudness.
When Spotify introduced their "Replay Volume Normalisation" in 2009, they put it at -12 LUFS. In 2017 they chose to change it to -14 LUFS (now 2 LUFS louder than the AES recommended -16 LUFS/-1dBTP) In 2018 Spotify introduced a new Volume Level feature that enables the listener to set the playback level based on their surroundings. Quiet - Normal - Loud. In addition to Off, that gives the listener the option of 4 different playback levels. Spotify determine the loudness level of the track as ReplayGain and is not a LUFS measurement, but measurements of a selection of tracks shows that the Quiet level is now approximately -22LUFS, Normal -14 LUFS and Loud -11 LUFS. To bring up the level of quiet masters (-12 LUFS and down), Spotify currently use a built in limiter.
My approach has always been to try to find the sweet spot for the level and amount of dynamics for any given track and aim for perceived dynamics and energy rather than measurements. I do try to keep up with the current playback levels of the different streaming platforms, but since it's a constantly moving target, I try to focus on making the track sound as good as possible within a dynamic range that's optimal for their target audience.