For a long time, the "itunes.apple.com" domain was used to preview releases available for purchase on iTunes. Since the advent of the Apple Music streaming service, the domain has been repurposed to list both releases on iTunes and streaming on Apple Music; in most cases this is not an issue since many releases are available for both streaming and download, but there are a handful of releases that are streaming-only. (I have not yet located a release that was download-only.)
Examples: https://itunes.apple.com/us/album/id1233641551 is not available for purchase on iTunes; it is only available for streaming on Apple Music.
https://itunes.apple.com/us/album/id1244418735 is available for both streaming and purchase.
If a release is available for both streaming and download, there will be a blue text blurb/link that reads "Also available on iTunes/Apple Music." Releases that are only available for streaming will lack this link.

UPDATE: As of 2019, "itunes.apple.com" links for releases redirect to "music.apple.com". There are several other tickets in the pipeline to address this, but in the meantime I move to repurpose this ticket to add a separate artist-level Apple Music relationship (example: https://music.apple.com/us/artist/393703) similar to the relationships we have for Bandcamp and the now-defunct CD Baby store.
Old iTunes artist links still work (for the time being, at least) so they should not be deprecated yet. I laid out a reason to keep them in MBBE-7; since Apple updated the music.apple.com interface on April 17, the pages only show content available in the region in which you are located (based on IP address, similar to Spotify, Deezer, Google Play and the like). It is still possible to preview region-locked content using an itunes.apple.com artist URL; the individual album links may not be followable, but it is still possible to copy the links and paste them into https://etc.marlonob.info/atisket/ for easy MB import.
Update: As of 2021-03-25 (likely earlier), iTunes artist URLs automatically redirect to the Apple Music web player. The old TLD swap trick still works for viewing content intended for various regions.