This (https://sylvan.apple.com/Aerials/2x/entries.json) appears to be the new URL for all the screensavers. The new entries file includes both normal and HDR versions of each of the screensavers, though on non-HDR displays, the SDR versions look a lot better than the (converted) HDR ones [1]. Unfortunately, all the new screensavers are encoded using HEVC [2] so they can't easily be played on OS versions without native support.
Open question to anyone with the latest tvOS firmware sitting around: what happens if you grep it for `entries.json`? Do you get a different URL?
Obviously what I'm getting at is ways to try and find (or keep an eye out for) the latest URLs.