It can download Javascript (or other interpreted language, or config data), but you generally can't download and execute new compiled native code on iOS, due to iOS code-signing/executable-memory restrictions.

Also keep in mind Apple might penalize you for dodging the review process and shipping new features, etc.

(btw, one exception to the JIT rule is custom browsers for the EU)

Precisely. That has always been prohibited both by technical measures and by the App Store review guidelines. Especially on Apple, but also on Google Play / Android as well.