This is not my experience. Claude has been happily generating code over the past week that is full of implicit any and using code that's been deprecated for at least 2 years.

>> 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?

The rate of change has made defining "modern" even more difficult and the timeframe brief, plus all that new code is based on old code, so it's more like a leaning tower than some sort of solid foundation.

ES6 is 11 years old. It's not that new.

> "ES6 being too new to be standard knowledge"

Huh? It's been a decade.

Theoretically, if you could train your own, and remove all references to the deprecated code in the training data, it wouldn't be able to emit deprecated code. Realistically that ability is out of reach at the hobbiest level so it will have to remain theoretical for at least a few more iterations of Moore's law.