Given our discussions on ticket T99009 we should mark the mobile view API as deprecated.
The deprecation message should link to the mediawiki.org page which will be used to communicate this change.
Note: With a deprecation we can take as long as we want so this does not mean we need to mark a day it will be removed just yet but we can at least start getting feedback from users of the API!
MobileFrontend's mobileview module accounts for ~0.73% of Wikipedia's daily action API traffic (see P7873).
= Acceptance criteria
 A deprecation warning is added to the API response that points developers to the REST mobile-html API endpoint
 The deprecation warning says "This API is marked as deprecated. Please see https://www.mediawiki.org/wiki/Extension:MobileFrontend/MobileViewAPI for more information.