Browser-based JavaScript is also very stable if you stay away from the frameworks in the sense that stuff which worked 20 years ago mostly still works today (although much stuff has been added of course).
Frameworks like Flask are also an anchor of stability and thankfully haven’t changed much, at least in terms of API stability.
Like Crockford's "Javascript: The good parts", which is when people really started using it without utterly despising it, was published in 2008.
Also, before V8 came out, JavaScript performance was a joke, a loop of 100 items could hang your page for a second. Think that was around 2008/9 too.
The easiest example is that you used to have to create and add every single node of a table, for example, modify all the properties line by line, create a tbody, add that to the table, create a tr element, add that to the body, add a td element, etc., etc.
I can do all that in one line now.
And 20 years ago, XMLHttpRequest had only just been released and you were more likely to be doing dynamic updating with crazy stuff in iframes.
Let's not even get started on divs vs tables for layout...
I’m not sure those are things for a younger developer to focus on, though.