Is relocation assistance offered to postdocs?

It varies. In my experience some of the more prestigious postdoctoral fellowships sponsored by universities might have some amount of moving expenses built into the budget, and more typical postdoc positions funded out of grants probably won't. But it might be possible to get a professor to help you out, perhaps out of startup or some other funds that aren't earmarked for a particular purpose. You should try to negotiate it with the person who offers you the job before you accept the offer.

In disciplines that I'm familiar with, postdoc money comes from a grant obtained by a PI. So the university has little-to-no role in the hiring and compensation process. I imagine the postdoc would have to discuss this issue with the PI.

In the early 90's, when I did a postdoc for a U.S. National Laboratory, full relocation expenses were included, which involved moving my household clear across the country, and staying in a hotel for a bit at the other end while I looked for a new home. I doubt such generous deals are readily available today.