It's immigration (and emigration) whenever a group of people migrate from one region to another, regardless of what the reason is or how they're treated.
It's a little bit of a tricky word territory because it would be inaccurate to call them "immigrants". That word is usually used in modern English to refer to non-forced migration, so could make the reader draw inaccurate conclusions.
It is, though, completely reasonable to put the event under a discussion of "Patterns of Immigration", because that is clearly referring to large-scale movements of people with important sociological and historical impacts. Historically, many major human migrations have been the result of slavery, exile, genocide, and other such unpleasant and rather non-voluntary reasons. They're still called migrations.