I once saw a youtube video which said that an object moving at 4m/s would be slowed down by [thing] by 0.00006m/s, so its new velocity would be 3.99994m/s

... except that no, its new velocity would be 4m/s. The author, a physicist, ignored sig figs. 4 - 0.00006 = 4, when you're dealing with measurements :)

sig figs are just an informal way of dealing with uncertainty that can be applied procedurally by high school students.

It's not about uncertainty, it's about precision. Unless, of course, you think that scientific notation is limited to only high schools.

What do you think the difference between precision and uncertainty is?

'Uncertainty' is a much more ambiguous term that means many more things than 'imprecision of measurement'.

Still, I'm interested to hear why you think sig figs are high-school only when they're the standard way of writing in scientific notation. Do you genuinely think that the example I gave from the video is reasonable? It sounds like you're a theorist and have no experience in real-world application.