As perspective on this topic, I had the same need years ago in my MC music library, so I created an workaround using Keywords for multiple Genres, and using custom field Artists to list all the notable performers on a recording. Comparing this "perfect" (for me) solution to the new changes...
Allowing a list for Genre is nice, lots of music fits into multiple genres. I'll likely adopt it for the main genres, but still use Keywords for many other ways I categorize tracks.
But using a list for Artist seems like a can of worms, for several reasons, starting with the potential for semi-colons. (I'll search for semi-colons in this field, just in case...)
I don't think the use of a list of artists is as "special case" as it might seem. In my library of 80K tracks spanning every type of music, more than half are appropriately tagged with multiple artists.
My solution, devised years ago, is to use a custom Artists field to list all performers. I get the same result but with complete control and precision. And, I still have an important use for the default Artist field, which I hope the new change won't mess up.
I use Artist as the field that contains the exact artist name as stated on the original recording. This is then displayed when the track is playing. I don't use it to group, sort, select or whatever, just to show the listener the correct and complete name of the performer/group. For instance, across a few years of recording the same singer and band might be listed on record labels in multiple ways; my use of Artist lets me show the correct name for each recording. (Example: Benny Goodman & His Orchestra, Peggy Lee Vocal; Benny Goodman and Peggy Lee; Peggy Lee with Benny Goodman & His Orchestra.)
I then use custom field Artists to list all the performer names in a standard format (the same for every track the artist is on, regardless of the artist/group as originally written), so I can directly select all the recordings by this artist. (For example, tracks with any of the different Artist names above would all be tagged as "Goodman, Benny; Lee, Peggy". Another example: Paul McCartney, Wings, and The Beatles when Paul is lead singer are all Artists tagged with his name, "McCartney, Paul", but also tagged "Wings" or "Beatles" as appropriate.)
My Artists field has all names as Last, First (group names are also standardized to always be the same, and omit articles), mainly accessed in a custom Artists view that top-level groups by first letter. As a list-type field it's easy to tag, selecting multiple artists as desired. For example, The Sound Of Silence is tagged as "Garfunkle, Art;Simon, Paul;Simon & Garfunkle" -- so the same song appears in my Artists view under all three names. This is enormously useful in every music genre, because artists often performed in a variety of groups and even with a variety of names.
Please note that I am reliant on custom list-type fields (I use several) and efficient keyboard tagging (my library will be growing by many thousands of tracks, ripped from vinyl, all added and tagged manually). Manual adding and tagging of this magnitude requires very efficient keyboard work, so I do all my music tagging with MC 15.0.122, in my opinion the last keyboard-efficient tagging version of MC. (The next version added forced-search to tagging, which drives me crazy; I can't find any way to be fast and efficient with it. My tags don't have anything I need to search for; I just want to directly jump through the complete list by typing, then select. Pane-tagging is not practical for me with more than 30 fields per track, and also not keyboard friendly.) I use MC17 for music playback, for photos (but would prefer MC 15.0.122 tagging behavior), and for video.