All speculative, but:

Part of the IQ test I took years ago involved memorizing and repeating sequences of digits. When I was growing up in the 80s, I had a large number of phone numbers memorized for friends, family, businesses I needed to contact regularly, etc. Presumably, memorizing dozens of seven- and ten- digit numbers regularly improved my ability to memorize long strings of digits.

Today, I no longer know any phone numbers besides my own and my wife's. And I know hers only because it's just one digit different than my own. I'm sure children are not getting any practice at this sort of thing.

So, does memorizing strings of digits have anything to do with how smart you are? If this is still part of IQ tests, then I'm not surprised raw scores are decreasing.

Also: Who's more intelligent, the person who spends time and energy on memorizing phone numbers or the one who buys a notebook?

Working memory is tremendously useful for a wide range of intellectual tasks.

Imagine a CPU with an eternal spinning-rust hard drive and no cache, vs a computer with a 10x slower CPU and a 4MB cache. Which do you thing is more powerful at solving problems?

Of course working memory is important and should be tested in an IQ test. But so is being able to use tools to augment yourself and work around your weaknesses.

And having a specialized ability to memorize numbers is quite useless.

That might be part of the overall testing you did in which more than just IQ was being testing. In a normal barrage you might get tests for working memory, attention, etc. which are not IQ. This sounds like it’s testing working memory.

This is the modern standard IQ test: https://en.wikipedia.org/wiki/Raven%27s_Progressive_Matrices

I don't have domain expertise here, but I suspect one example would be simple numerical calculation: electronic devices are so widely available today that hardly anyone can do long division in their head, or by hand for that matter, whereas relatively many more people in 1970 probably could, and had no ready electronic alternative. Whether people in 2019 could learn this skill as well as people in 1970 if it were deemed a priority, I don't know, but I think it's fair to say that the skill was much more valued in 1970, and considered a more integral aspect of "intelligence." (And of course, numeracy only became so important relatively recently in history -- centuries ago, the ability to successfully hunt and forage was much more essential to most people's definition of "smart" than anything involving numbers.)

I’ve not seen any numerical calculation in an IQ test I’ve done that isn’t in some form just a puzzle and that doesn’t get more advanced than basic arithmetic. Being able to reason about numbers, addition, subtraction, multiplication etc. is perfectly in line with measuring intelligence.

Correct -- I took the parent question to be about shifting cultural definitions of "smart" and not about IQ tests specifically.

People in 1970 had to navigate with paper maps, which is actually a cognitively loaded activity.

People in 2019 do not, so they don't get the same amount of spatial reasoning exercise from that activity.