Most content creators still upload 1080p because of the smaller file size and shorter upload time.
The rest use 4K.
If I had to guess, less than 0.1% of all content on YouTube is higher than 4K, roughly. Even channels like Linus Tech Tips that I know for a fact have 8-12K cameras only upload 4K.
5K is not a common recording media format. The few consumer phones that do above 4K jump straight to 8K. Professional cameras typically record in 4K, 6K, 8K, and 12K. Usually they are recorded in higher than 4K resolution to master down to 4K. It's most useful for post-processing image stabilization, cropping for zoom, and for reducing low light noise through downsampling. Any larger image downsampled will give you a better result than an image recorded natively at the resolution of the downsample. The result is always sharper due to the way that imaging sensors record data and how that data is compressed.