I’m curious, why is it awkward to write with a regular Russian keyboard layout? Presumably due to punctuation and not due to the Cyrillic characters?

Some symbols may not be readily available, e. g. <> or {} or [] and you'll have to either switch to English or install and learn a modified layout. But this is mostly a Windows issue, I think; the Russian keyboard layout on Windows is barebones compared to that of Mac. As a result there are other custom-made layouts in circulation, e.g. a typographically enhanced one.

I've been using Mac for ages but I use the layout Apple calls "Russian — PC". These characters are still not really that available. There are some alt-shift-whatever contortions you can do to type them, but it's easier just to switch to English and back again. Apple's default Russian layout is even worse at the very least because the period is shift+7 instead of the key between Ю and shift.

I think I still have a book on C-in-cyrillic somewhere. Like, just all keywords just translated to Russian.

I've seen a meme where someone did that with a bunch of #define's

Meme's a meme. That is a hardcover, printed in at least 10s of thousands.

So is it really a book about something like

    #включить <стдвв.з>
    
    цел главная() {
        печататьф("Привет, мир!\н");
        вернуть 0;
    }

Yup.

There's also Kumir, which is an educational programming language used in Russian schools

Hm. That must be new, I was taught Turbo Pascal

Somewhat new, or at least wasn't used in schools until fairly recently. It's a programming environment with tools like Turtle Graphics built in, specifically for teaching the basics of coding. There are even some tasks in ЕГЭ for it.

https://www.niisi.ru/kumir/

The website screenshot shows it on Windows XP though, don't know if it actually existed back then or if it's just typical Russian institutions still using Windows XP.

In fact, the language itself dates to rhe 1980s I think. No idea whether its use in schools is recent though.

I had to deal with 1C once for a client who insisted on reconciling his (mid-9 figure) assets into it. The good part of it is that a competent 1C programmer (of which he had 2) can basically make it do anything, exactly like SAP, but the out of the box experience is terrible.