This has been common parlance in much of the US for a long time. I would hesitate to even call it slang at this point. It's a pretty commonly used term.

Interesting question. I've never heard "relocators" used in this context, only "transplants." And I am familiar with that usage across cities etc.

"Transplants" is a common term nationwide.

In Oregon, we often refer to people moving from California as transplants.

It's not specific to SF but more or less yes

Lots of transplants in Colorado too.