I updated my iPhone to iOS 11 over the weekend, having first replaced or exported data from two old apps that haven’t been updated for 64 bit (I’ll miss you, Cocktails app!). And then I synced music from my Mac and noticed that I didn’t get the customary message about tracks that couldn’t be synced.
I checked and found that a whole bunch of Boston Symphony tracks purchased from their store, which I converted from FLAC to Apple Lossless but were apparently still at a too-high bitrate for iOS to handle, appear finally to be supported and were synced to my phone for the first time ever. This appears to be a feature, and may be related to the ability to play back FLAC through some apps (like iCloud Drive).
Looking forward to finally carrying all my music with me!