Currently, the proofreading status is stored in the Wikitext of the Page namespace main slot content. This is undesirable because it makes it awkward to present the page in various editors and binds the content and metadata uncomfortably.
Moving the proofreading status to a multi-content-revision (MCR) slot will allow us to keep the revision data in the revision, but also allow us to separate the content and status.
By using, e.g. a serialised JSON format, it will also be possible to easily add more data to the page metadata in future.
This is a separate issue to using change-tags (T290578) for a query-able and filterable view into page histories, which are currently derived from the <pagequality> tag, but moving it to a new MCR slot will not affect them.
See T290578#7357138 for more comments to this effect.