There are things I used to understand while studying that I no longer do, but wish I did. I can look them up and understand if I want to intentionally use them, but they're not sort of available in my mind for me to draw on; information that might apply to something without me knowing in order to look it up, if that makes sense?

And also as a sibling comment says, language. In particular vocabulary - I've learnt a decent amount of Hindi grammar by self-study, in fact I think totally sufficient for casual or even professional conversation, but my vocabulary's really lacking, and it holds me back from speaking, which means even when I do have the opportunity to practice speaking with others, I can only say stupid basic things, I lack the vocabulary to say what I want most of the time. (And casual Hindi's way more forgiving than most languages, because 'Hinglish' is so common. But I don't get any better by just using Hindi connectives and ancillary words in an otherwise English sentence!)

I'm studying data structures and algorithms, out of interest, and to pass technical interviews. I can't fire up ChatGPT during interviews.

Anki has definitely helped me. It's surprising how well it works if you do it daily.

I also add new cards when I come across a word I don't know. If I keep this up for years or decades, my vocabulary should be extraordinary.

When learning a language, yes!