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Some YouTube Premium subscribers find that they have an additional “1080p Premium” option in the image quality menu, and there is a descriptive text below the option that says “Enhanced Bitrate (Enhanced Bitrate)”. According to a YouTube spokesperson to The Verge, this is a new feature that YouTube is testing and is currently only available to a “small percentage of YouTube Premium subscribers.” The spokesperson described it as “each pixel delivering more information for a higher-quality viewing experience.”
YouTube also stated that the original 1080p standard resolution has not been changed, but the Reddit discussion thread does not seem to believe it, thinking that the standard resolution 1080p video seems to have downgraded the quality. However, such things as picture quality are often quite subjective, and factors such as how the increased bit rate is applied and the picture quality of the original uploaded video will also affect the final viewing experience. Is the original 1080p available? How much better it is to be tuned, or the high bit rate 1080p, probably only YouTube itself knows. In addition, if the video also has a 4K (2160p) option, even if the frame size is the same, 4K should be sharper, but the file size will be much larger. At this time, the high bit rate of 1080p may be a compromise choice.
Of course, the current high bit rate 1080p is still being tested, and it is not certain whether it will be rolled out to all subscribers in the end.