Take one example: A program called SchrödingersCat needs a command line flag to specify if the cat is alive or dead. But when called with a value of "undef", ...

A nice example, indeed. Just one question: How do you pass an undefined value as a command line flag?

See Tom Scott's video....

Thanks: Another example which underpins my point. The video explains the difference between NULL, "NULL" and 0 when read from a database. So yes, a database has NULL as the equivalent of an undefined value. But a database has no notion of "this field does not exist": If a field is in a table definition, then it exists for every single record. So, again, it has only one special value, not two.