Earlier quoted context omitted.
By far the most useful and helpful is job ads: it literally defines the demand side of the programming language market. Yes, that does not show us how much code is running out there, and some companies might have huge armies with very low churn and so the COBOL stacks in banks don’t show up, but I can’t think of a more useful and directly measurable way of understanding a languages real utility.
Yeah except job adverts have enormous lag behind what's actually popular. For example we used Rust quite a lot at my previous company but we didn't advertise for Rust developers at all. Also then you're looking at which languages were popular in the past whereas the interesting stat is which languages are being used to start new projects.
If I'm trying to figure out which language to learn next, knowing what I can get paid for might be more useful, even if it's not that "interesting".
If lots of projects are starting up in Rust, but I can't get interviews because nobody is advertising, how useful is learning Rust?