[randomdata]

If someone walked away 15 years ago, what would they have missed if they decided to come back today?

I honestly can't think of anything fundamental. Mobile and single page web applications, for example, became popular in that time, but those only brought back the same patterns we were already accustomed to on the desktop, not some brand new paradigm that someone from 15 years ago could have never imagined and cannot contemplate now.

Perhaps you might have to spend a week or two catching up on the specific keywords used by the popular framework du jour, but that's not a hurdle in any meaningful sense.

Granted, someone from 15 years ago coming back to an "Agile" shop will run right back where they came from, as fast as they can, wondering what on earth we are thinking the whole way.

[AstralStorm]

Worst case it's a year of a crash course to learn almost anything in enough depth to be proficient, even if not quite an expert. With the base in programming or electronics, it would take even shorter time with a well designed course.

[randomdata]

What would justify a year long crash course, assuming you have a functioning memory? I still cannot think of anything that has actually changed, in a fundamental way, over the last 15 years.