Yes, universal. Many websites, apps, communicators etc. implement a flavour of TeX. They will differ in some more complex features or commands, but your $a^2+b^2=c^2$ will work. And that’s the point. For most times, you’ll just want to communicate some simple concepts.

We could do better, for sure. For example, there’s been some development around Typst, which tries to resolve many of those quirks, annoyances, and inconsistencies that come with TeX. It makes sense, since TeX evolved rather naturally and outgrew what it was initially comprised to be. While I’d love to see Typst come far, for now TeX is something that I take for granted, which perhaps better encompasses my thoughts than the word “universal”.