A system I work with gives all keys a string value of "Not_set" when the key is intended to be unset. The team decided to put this in because of a connection with a different, legacy system, whose developers (somehow) could not distinguish between a key being missing or being present but with a null value. So now every team that integrates with this system has to deal with these unset values.

Of course, it's up to individual developers to never forget to set a key to "Not_Set". Also, they forgot to standardise capitalisation and such so there are all sorts of variations "NOT_SET", "Not_set", "NotSet", etc. floating around the API responses. Also null is still a possible value you need to handle as well, though what it means is context dependent (usually it means someone fucked up).