The first time I noticed the problem was several years ago while listening to The King's Speech sound track spotify:album:6VKvELAackQ5QfVscXz7It
You can clearly hear the flutter. No, those strings are not supposed to have that aggressive vibrato or tremolo. There should be a tiny bit of bounce in the bass but that's it.
They still haven't fixed the problem. I finally managed to contact Spotify support with a long list of albums and tracks with the problem, from Mozart to Hans Zimmer. They escalated and basically told me they knew about the problem for several years (Montag was one of their employee after all) and didn't give me any information about whether they would fix the problem or not and closed the ticket.
My totally speculative conclusions: Either 1) Spotify don't care, despite people paying premium for high quality streams, or 2) UMG hate Spotify (they actually do) and refuse to reupload the library on purpose.
I don't remember where I read that. It might have been on a forum with people complaining about their lossless downloads being affected by it. I do remember they had a link to the 3rd party company responsible for that.
As for the timeline, 2013 is a total guess of mine simply because music released after 2013 doesn't seem to be affected.