I noticed that the paragraphs about constructing the release identifier are still in the core spec, but concatenating the two values is not an authoring concern.

These paras need to migrate to the reading system specification, but should we make an (informative) appendix out of them? Even if a reading system creates and uses release identifiers (and I'm not sure they do), there's no requirement to. It's sort of a helpful idea on how to distinguish publications more than an actual requirement of the specification.