[_fizz_buzz_]

It obviously never became completely irrelevant. But I think programmers spend a lot less time thinking about memory than they used to. People used to do a lot of gymnastics and crazy optimizations to fit stuff into memory. I do quite a bit of embedded programming and most of the time it seems easier for me to simply upgrade the MCU and spend 10cents more (or whatever) than to make any crazy optimimzations. But of course there are still cases where it makes sense.

[II2II]

While thinking less about memory optimizations is possible since we have more memory, it was enabled by the languages and libraries we use. Fourty years ago, you were probably implementing your own data structures. Sure, there were plenty of languages that offered them back then (LISP was based on linked lists, and that language is from the 1960's). Chances are you weren't using such languages unless you were using big computers or writing software that didn't handle much data. These days, pretty much any language will provide at least some data structures and their related algorithms. Even systems programming languages like C++ and Rust. Of course, there are an absurd number of libraries if you need anything more specialized.