I'm not very sure about this one, migrate refers specifically to the migrate module which serves the purpose of data migration but isn't the only tool useful for achieving said migration. I think it's necessary to keep this tag for questions about this module.
data-migration will cover much broader topics than migrate.
Why should these be synonyms? We generally only want to create synonyms when there is a demonstrated need, e.g. people are asking questions with those tags.
Doing it speculatively would lead to a lot of useless mind-reading synonyms...