ColdFusion! It's been dead longer than I've been developing with it; but is still going strong. :ha, ha:

Seriously, though, I would not say there are any core technologies; but rather core programming concepts.

Variable memory management is one example. Knowing the difference between passing parameters by value, or by reference, when writing Lotus Notes applications actually helped me understand the difference between a variable assignment and creating a deep copy in JavaScript.

Less specific; If I had to guess--in 20 years; I'll still be working in technology that supports things like MVC(S), Dependency Injection, Singletons, Factories, etc....

Learning basic encapsulation techniques, and design patterns, is knowledge that will apply to lots of programming languages.

Unix, git, networking, C, HTTP/HTML/CSS/JS, databases, blockchains, data structures, compilters, most things in a CS curriculum, etc...

I saw an amazing presentation at a ruby meetup about database indexes for developers (for example). Will see if I can dig it up.

Here you are. The slides aren't as good as the tall (as you'd expect) but still may be helpful.

Other core technologies (in my opinion): cookies, http, TCP/IP, caching.

Ack, forgot the link: https://speakerdeck.com/heimir/databases-for-developers