It's a "cinematic" aspect ratio. I can't tell if Youtube is simply not adding in the bars they used to use, or if they're intentionally pulling them out to fit their new adaptive design.
For dream-induced lucid dreaming, the most commonly recommended beginner technique is the re-reading strategy. You read text, remember it, and then read it again: In a dream, it will always change. The same goes for numbers on a digital clock face.