From context, I think it just means "off-camera flash", or "remote flash".

I think this is an accidental import from French (le flash déporté), and not a common term in American or British English photography usage today. Particularly, the person who used it on this site, prompting this question, is a native French/German speaker. And, many of the quotes Rob found also appear to be from native French speakers. That in itself doesn't mean that it isn't legitimate English, but it may be more something common to French speakers of English than to American or British English.

Meanwhile, the phrase "deported flash" is not found at all in a Google Ngrams search. I did find "deported ring-shaped flash or with deported satellite flashes" in a book about finger- and toenail diseases (warning: gross!) — but, hey look, at least one of the editors of that book has a decidedly French name.

Of course, English doesn't have any ruling body dictating what words are legit or not, and we borrow words from other languages with gleeful abandon. So, it's possible that this is emerging usage, or will be soon. To me, though, the word has decidedly negative connotations, and particularly in today's political world with migration being a sensitive topic, I don't think it's likely.