unfortunately, I have a feeling that in the age of LLMs, this junior on the team will have no impetus to actually put in effort and _think_ about such a problem
Because SO at least requires you to THINK a little about what do you have a problem with.
With LLMs, you don't even need that. Just copy paste the error and you get a response. Copy and paste the Jira ticket description and you get a response. This wasn't possible with SO. Yes, none of those will likely work straight away but the point is that less thinking is required.
Hopefully, the junior's code will be reviewed before it gets merged
Did we survive the age of StackOverflow though? The market (globally) is absolutely flooded with not-even-mediocre software devs who are effectively doing what an LLM is i.e. finding the most plausible looking answers on SO and somehow munging them together, without any real understanding of what they're doing nor why it's working (or not working). The number of people charging contract rates yet lacking an understanding of actual software design principles (largely language agnostic) and no idea how computers actually work, is scary.