As can be seen, channel groups and the channels grouped under these can now get displayed using different colour sets (this colouring can be turned off via the View menu for a more subdued experience). The eventual goal is to make it so that these channel groups can take their colours either from the colours used for a corresponding Bone Group, or the colours specified in the Keying Set. (The colours seen in the screenshot were set manually via RNA).
While perhaps still not as good as being able to collapse down groups of channels for related bones to work on them (I must dig out a draft post on the technical considerations behind this that I've had lurking around for ages), at least this makes it possible again to see visually what bones belong to which "cluster". Who knows, we could add in colour-based filtering options!