My experience is that they somehow print quite modern code despite things like ES6 being too new to be standard knowledge even for me and I'm not even middle-aged yet

Maybe the last 10 years saw so much more modern code than the last cumulative 40+ years of coding and so modern code is statistically more likely to be output? Or maybe they assign higher weights to more recent commits/sources during training? Not sure but it seems to be good at picking this up. And you can always feed the info into its context window until then

Which is why it can easily understand how innerHTML is being used so that it can replace it with the right thing.

Honest question: Is there a way to get an LLM to stop emitting deprecated code?