Suggestion: add metadata field for marking a proposal as outdated / merged into upstream spec

For proposals that have reached stage 4, it seems worthwhile to show a big warning banner at the top of the rendered spec text, urging folks to no longer reference the proposal itself but instead link to the integrated version into the spec text. Something like this (but with better phrasing):

Note: This proposal has been accepted into the upstream ECMAScript specification — please update your references to point there. This snapshot is only kept for historical purposes.

For styling inspiration, here's a similar example for a CSS spec: https://drafts.csswg.org/date/2018-07-09T23:26:30/css-box-3/

Currently, proposal authors can manually implement something like this, but it would be helpful if there was a standard mechanism to accomplish this.

We could either introduce a new metadata field (maybe deprecated: true?), or we could do this automatically when both status: proposal and stage: 4 are set in the metadata.

cc @tc39/ecma262-editors

Start with the ecmarkup document options described in the linked specification and inspect how proposal metadata is currently rendered. Decide whether the banner is triggered by a new metadata field or by the existing status and stage values, then define the wording, styling, and expected behavior for accepted proposals.