There’s two things mixed up in this thread (for which there was further related discussion recently at Playlists?), so to clarify:
Indie/Rock “Playlists”
As brought up earlier by others including @aerozol and @teethfairy, it seems the Indie/Rock Playlists specifically are pirate/bootleg downloadable crap (opinions my own), and as such, thinking of them as Spotify playlists is misleading; they seem to just use Spotify and other similar sites as a (legal) alternative to the download since that’s an option now.
The question for these is: “is a site putting together a large pirate file every month a legit MB bootleg release”? I don’t know the answer for that, really (I would not add them but they seem popular enough to be equivalent to a bootleg CD series).
Personally I’d love to make a style call that anything like this is banned and should not be added (I don’t particularly like aiding piracy while making editing harder for everyone else) but we should have reasonable community near-consensus to make that call. At the very least though, the links to piracy should not be added, including as edit notes - feel free to report any editors adding those and I’ll talk to them and blank / edit the relevant edit notes. If they keep adding piracy links I’ll have to block them, but hopefully they will stop that practice.
Actual streaming-only playlists
There’s certainly cases where a playlist is actually a release in all but name. For example, some Spanish rap music used to be only put out as a playlist of YouTube videos named as an album, with cover art to boot (not sure if those now end up on YouTube Music also as more “legitimate” releases?). Cases like that obviously belong in MusicBrainz, and as such YouTube playlists are not blocked for MB releases.
For most streaming sites, in order to be able to use a song in a playlist it needs to have been added before as part of an album or single or whatever. In those cases, there’s always a more legitimate release than the playlist to add if you want to add music missing from MB. For this reason, Spotify playlist links are currently blocked from releases. I agree that it’s theoretically possible for a label to stop putting out sampler promotional CDs and just make playlists of their music on Spotify and share that. Those promotional releases were IMO barely worth adding to MB (mostly having value only inasmuch as they were a physical collectible thing). As playlists, their value seems to be zero, and I’d suggest just adding the actual music releases they promote. Since they’re not downloadable (unless they have an associated bootleg download, in which case see the first section instead) then there seems to be no use of them in Picard and similar taggers and as such zero reasons why they should be a release over a series (arguably in most cases it’s not worth it for them to be either).
If we find edge cases of playlists which do seem like they should qualify as a release, we can debate if an exception should exist and on which conditions.