Do universities offer relocation assistance to a newly employed postdoc who comes from another country? Is it appropriate for the postdoc to ask for relocation assistance if it is not already offered?
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 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.
In the Netherlands, relocation reimbursement for post-docs, both a lump sum and a refund for transporting your possessions, is mandated by the collective employment agreements. For the non-university research institutes, Chapter 10 of the the 2018 CAO agreement spells this out. The amount is about 2,000 EUR for a fixed-term employment contract.
I have no experience elsewhere, (un?)fortunately.