It amazes me the impact YouTube has on my hobbies and curiosities now. I can’t think of anything you couldn’t learn how to do in at least a beginner/intermediate level. Can you?
Just in the past couple months I’ve watched how to vids on:
How beekeepers harvest honey
How to measure for curtains
How a pro photographer sets up to do Sr portraits
How to install a window
How to weatherstrip doors
How to measure fabric for a quilt
How to use a pressure cooker
How to write with a glass pen
How to build cheap shelves
I remember when you used to have to take a class or know someone who knows already. It’s miraculous!
Is there anything that’s still a mystery, future lost art?
I’m pretty sure that many activities such as snow skiing, snow boarding, driving a car, etc. one could not learn to a beginner or intermediate level, by just watching a video.
Beginner snow skiing
I think I could learn enough via video for me to know how to do those things, but yes knowing how and having experience are not the same things.
I think it depends on who you are. I can only learn how to do something physical by doing it; copying someone else is almost impossible for me. So youtube is not much use to me that way.
I learned how to harvest honey by working with a beekeeper. I learned how to measure fabric by measuring it and cutting it wrong a few times. And so forth, with all the things I know how to do.
Is there a how-to-do cunnilingus video on YouTube? 
mm mm mumble mumble mm mmm.
since anyone can put up a video you have no way of knowing if the advice is good. totally wrong and dangerous procedures might be suggested. even if not totally wrong there may be details left out or wrong that would give a less quality outcome.
good to look for people claiming experience in the area either as a trade or accomplished hobbyist.
I was thinking of expanding into laser prostate surgery after watching a YouTube video (my state license says I’m authorized to practice Medicine and Surgery). But just running an ad in the paper saying I’ve watched a really good YouTube video probably won’t get me a lot of patients.
People are also pooh-poohing my planned shortcut into neurosurgery.
You asked for it.
Seriously though. Obligatory NSFW mask