As in, will we come across a lot of non-semantic HTML in places where semantic HTML could have been used, purely because it wasn’t industry standard to use semantic HTML in the past?
And, is it encouraged to edit historic non-semantic HTML tags to become semantic, even if you were not the original author? What are the rules around this?
Hi there!
Welcome to the forums!
Semantic HTML is important for accessibility, user experience, SEO, etc, so all modern websites use (or should be using) semantic HTML. However, you will come across a website now and then that hasn’t been updated in a long while. (I redid an entire website for a business that hadn’t had their website updated in almost 20 years 
)
So, I would absolutely encourage anyone to edit historic, non-semantic web pages/tags if given the opportunity and to ignore anyone who says otherwise. 
Thanks for the info @kirativewd ! 