I believe this is okay to remove because:

• For cases where this is transmitted over a versioned JavaScript response, even if a site upgrades from MediaWiki 1.23 to the latest release in one go, it will be fine since our version manifest is in the same reponse as the loader client, which enables forward-moving cache propagation (Docs). Basically, any client with the old manifest has the old loader and request older versions. Any client with the newer manifest, has the newer client and thus bypasses any cached responses from before.
• We do sometimes have to account for backward-compatiblity, namely in our unversioned responses. This is mainly the case for stylesheets for the skin, and the cached HTML. However, while this task is about compatibility code for stylesheets, it is specifically for JS-loaded stylesheets, not the stylesheets of style modules queued server-side.