It's back!
I skipped through ARRIs YT channel to look for 1080p videos that had it, and I found a couple of interesting things:
ARRI upload videos in all sorts of resolutions, 4K, 1080p, 1440p, even 720p, even recently
The 1080p uploads they did only have the Enhanced Bitrate option very recently - the 1080p ones uploaded more than a couple of months ago didn't have the option
This video has it:
Here's a comparison. This is the 1080p video displayed full-screen on my UHD display and then a screen-grab taken.
Standard 1080p:
Enhanced Bitrate 1080p:
Standard 1080p:
Enhanced Bitrate 1080p:
Oddly, it's not available on this 1080p upload, which is more recent than the above.
@kaylee Do you have it enabled yet?