Currently the Artist Credits style guideline makes minimal mention of collaborations. A few examples are given, but only to illustrate join phrases and capitalization of articles. The Artist guideline makes no mention of collaborations at all.

The How to Split Artists guideline is still marked as unreviewed, which I assume means it's not official policy. It does, however, state:

Please make sure the artist should be split before entering the edit! Established bands (like Emerson, Lake and Palmer) shouldn't be split; this is intended for short term collaborations that don't merit their own artist page.

...although there's no information anywhere on what constitutes a short-term or long-term collaboration. Emerson, Lake & Palmer is given as an example (which makes sense), but [Emerson, Lake & Powell] is also a single artist, even though they only released one album and were together for maybe a year or two.

The Theatre style guideline lists five very well-established, well-known, long lasting teams and says "The following commonly credited composer/lyricist collaborations should be expressed as joint artist credits to the composer and lyricist." There's no explanation why those specific five teams should have separate artists credits, and as a result, an artist search for any of them only turns up individuals, and there doesn't appear to be an easy way to see a list of their releases as a team.

Please provide a guideline with the criteria for whether a collaborative artist should exist or ACs should be used for multiple artists. Also please consider allowing a single MB artist for each of "The Theatre Five".