What does the toad that Deckard saves symbolize?
It symbolizes himself and humanity (both the quality and the race).
He can't tell reality from constructs any more, but he knows that something about the toad is precious. Perhaps it is the act of saving it and caring about it. However, the toad's value is real, no matter where it comes from. Likewise, his actions are real and matter, whether he is real or not, and whether they were programmed or not, they were his. (His actions are his as the toad's reactions belong to it.)