This has less to do with the technical merits of Plan 9 and more to do with the fact that nobody has ported a modern web browser to Plan 9 (there are some browsers, but none with the feature parity of Firefox or Chrome). A slightly easier route to YouTube support for Plan 9 would be if somebody ported youtube-dl, but this work is also non-trivial.
It’s the classic chicken-and-egg problem of adopting new platforms: the platform needs software to attract users, but developers won’t develop unless there are users or there’s a high likelihood users will come. It’s not that Plan 9 isn’t made for our world; it’s just that Linux/Windows/macOS/BSD and their software ecosystems are good enough for many users, and those who want to take advantage of Plan 9’s features have to contend with a lack of software in many domains. It’s similar to the reason I code in Python for my machine learning job despite the fact I’d prefer a Lisp or a statically-typed functional programming language: Python has a far richer ecosystem of machine learning and data science libraries.
If that is unsatisfactory then you can either A. use vmx(1) to run Linux and run a browser in there. or B. write code and contribute instead of complaining.
Though what does youtube have to do with modern computing? It's just an application. I'm talking about the underlying architecture.
Actually, there is (I spotted a YouTube downloader someplace), but besides that "point", the videos in that channel are _very_ interesting if you care anything about modern microprocessors (he's ported Plan9 to quite a few devices).
Unless I'm mistaken, there is no way to watch YouTube on Plan 9, which rather reinforces the point that it is not made for the modern world, no?