I think I’ve seen news rumors that they’re testing an ad-free YouTube-only plan that doesn’t include YouTube Music. (edit: https://www.theverge.com/2021/8/2/22605455/youtube-premium-lite-pilot-benelux-nordics-experiment-ad-free-subscription)

But the reason you’re seeing $18.99/mo is because of the Apple tax. Go to https://www.youtube.com/premium to see the actual price.