I am not sure what to make of John Mueller’s comments about PageRank related to using the old PageRank model for managing internal link structure. The exchange on Twitter asked Google’s John Mueller if the “original PageRank to model internal link weight have any value nowadays.” John said “It doesn’t have to be exact to be useful though.”
Here are the tweets:

It doesn’t have to be exact to be useful though 🙂
— John ☆.o(≧▽≦)o.☆ (@JohnMu) October 19, 2017
I guess what John is saying here is that it doesn’t hurt to understand the original PageRank model because even if it did change, the original premise behind the rational around PageRank is about how Google defines importance of a page. Understanding that concept for internal and external links is probably a useful thing to understand.
Forum discussion at Twitter.