[loup-vaillant]

I've learned one important thing on the job: how not to do things. That's more important than it sounds. I noticed for instance that I developed a taste for simplicity over the years that wasn't there when I started. Now junior programmers can read my code.

Domain specific knowledge (so I can talk to the customer) I generally learn on the job. But it's not generally applicable, to other domains, and thus other jobs.

Technical, generally applicable knowledge? Mostly on my own time. Companies don't want to pay you to learn. They want you to apply your knowledge. I suspect this explains in part why they can't retain employees: they learn nothing, they get bored, so they leave. Changing gig at least lets you learn a couple new things the first few months.