You are correct: the specification is a work in progress which we plan to wrap of quickly. The approach is not to over-specify the format with features/options initially, but begin with a functional sub-set of HIEF and MIAF. The target is to provide an HDR/WCG successor to JPEG. This work is being done by a working group in the Alliance for Open Media.
Since there are plenty of reasons you don't care about this metadata (e.g. to reduce the transfer size on images that are requested millions or billions of times by a web browser), as much as possible it should be left optional.
But all of this seems to be pretty much draftware at the moment, so we'll see how things shake out.