送返 is fairly formal and written - common in news, legal, and official contexts (deportation, repatriation, returning evidence). Spoken Chinese more often uses 送回 or 送回去.
usage
Used especially in cross-border contexts: 送返原籍国 ('sent back to country of origin'), 被送返 ('to be repatriated'). Hong Kong news uses it heavily for extradition cases.
