I might be missing something, but from what I see in includes/Rest/Handler/PageContentHelper.php and https://api.wikimedia.org/wiki/API_reference/Core/Pages/Get_page it does not seem easily possible to retrieve HTML or wikitext for a specific revision ID in the page HTML/source REST handlers. I propose adding an optional revision query parameter to PageContentHelper so that one could query for e.g. https://api.wikimedia.org/core/v1/wikipedia/en/page/Earth?revision=1025714312.
My specific motivation for this is that in the link recommendation service, I would like to make it to retrieve link recommendations for older content. This is useful when debugging how users interacted with the link suggestion tool in MediaWiki. As it is now, a user accepts some link suggestions and makes a new revision to the article Earth. If I want to understand which suggestion were shown to the user, I need to be able to call the link recommendation service using a specific revision ID for the article Earth. That's possible using a POST request (https://api.wikimedia.org/wiki/API_reference/Service/Link_recommendation/Get_link_recommendations_for_revision) that supplies the revision ID and raw wikitext, but that is not as straightforward as a GET request (https://api.wikimedia.org/wiki/API_reference/Service/Link_recommendation/Get_link_recommendations) which says which revision ID should be used by the link recommendation service.