Like with any kind of learning, without a feedback loop (as tight as possible IMHO), it's not gonna happen. And there is always some kind of feedback loop.

Ultra short cycle: Pairing with a senior, solid manual and automated testing during development.

Reasonably short cycle: Code review by a senior within hours and for small subsets of the work ideally, QA testing by a seperate person within hours.

Borderline too long cycle: Code review of larger chunks of code by a senior with days of delay, QA testing by a seperate person days or weeks after implementation.

Terminally long feedback cycle: Critical bug in production, data loss, negative career consequences.

I'm confident that juniors will still learn, eventually. Seniors can help them learn a whole lot faster though, if both sides want that, and if the organisation lets them. And yeah, that's even more the case than in the pre LLM world.