- Mastering is necessary to prepare music for physical media.
- Mastering is elective for purely digital distribution.
- Benefit of "mastering" digital music is subjective, and anything beyond raw volume is likely to go unnoticed by the average listener.
I'm not going to deny that record labels still send albums out for mastering. But they're also still manufacturing CDs and even vinyl records. And to the extent of what mastering engineers are doing for digitally distributed music, that has changed, and it is far less crucial than mastering for physical media.
The move to predominantly digital forms, starting with CDs, caused an identity crisis among mastering engineers, who were no longer primarily concerned with getting the music to sound as good as it could within technical constraints, so they started proving their worth by showing how loud they could make it within digital constraints. And that led to the loudness wars and a lot of ruined albums on major labels from some of the supposedly best mastering engineers in the industry.