Google performing A/B test for new YouTube Music "In Library" search feature
A Reddit user with the name PeepAndCreep (via AndroidPolice) has discovered what appears to be an A/B test for an "In Library" filter for search on the mobile and web variants of YouTube Music. Type in your search request and YouTube Music will scour your library and post the results. Keep in mind that the results won't show uploaded songs, artists and albums, only those items held in your YouTube Music streaming library.
After playing around with this discovery, PeepAndCreep discovered that if you click on an artist's icon from the results list, you'll be whisked away to that artist's "In My Library" page. On the other hand, if you tap on an album from the results page, you will be sent to that album's YouTube Music page even if you just have a couple of songs from that album saved in your library.
YouTube Music tests an "In Library" search filter
Since uploads won't show up in the filter, those who play only uploaded music will not be able to search for their own playlists. Additionally, playlists you don't own, official YouTube Music playlists and those created as part of a collaborative effort that you didn't start will not appear in the search filter even if you have editing access. The Reddit user also noticed that songs featuring more than one primary artist will appear in the search results only when the search is conducted under the name of the first listed artist.
PeepAndCreep (Mr.Creep?) is pretty sure that this is an A/B test since the new feature appears on his main YouTube Music account (both web and Android) but doesn't on his brand account. Search on the mobile YouTube Music also won't work if you're offline.