As of DM-13911 there are now multiband versions of the afw Image, Mask, MaskedImage, Exposure, and HeavyFootprint classes called MultibandImage, MultibandMask, MultibandMaskedImage, MultibandExposure, and MultibandFootprint respectively.
The behavior of the multiband classes is similar to their single band versions except that the first index is now in the filter dimension, so that slicing an image with filters (“G”, “R”, “I”, “Z”, “Y”) is performed using
image["G":"I", bbox]
For a more detailed description of the new functionality and behavior see the docs.