I've given up on hoping each Gutenberg release will have a dedicated metadata API or at least some simple implementation of taking some of the important (to me) php developed metadata components I've customized for my site over the years. Your prior post about the Fields API was encouraging but that seems DOA.

Textarea controls with html/Tinymce editing capability to store metadata and any kind of AJAX related population of html form control options/values are nightmare fuel. I understand most users want easy to generate polished content and rarely use metadata but for me, I can't use Gutenberg to do the same stuff I did in classic without some serious workarounds and learning investment.