I tried adding type hinting to a program that I had to maintain once - because it had no unit tests and I was looking for the quickest way to make it less fragile. One tended to make a small change and then find that it broke something somewhere but finding that out was a long process.

FWIW I found it nearly useless - whereas biting the bullet and refactoring it so that it could be unit tested - even just a few tests - made a world of difference.