Who says they can't say "slavery"?I think you are missing the point. No one is objecting to using the phrase “involuntary relocated”, they are objecting to using this phrase in the place of “slavery”. The point of this is that they are not allowed to say “slavery”.
Involuntary relocation was one aspect of the Atlantic slave trade. Capture by indigenous populations, selling at African slave markets, involuntary relocation to America, another sale at American slave markets, and finally forced servitude.
And it is an important one to learn about. The hellish conditions, the treatment they received, the absolute debasement of humanity, the denial of dignity, etc.