Well, I'm not sure it's fair to say the number itself is illegal: it's the code that breaks the law, the number just happens to be one way to express it.
A perfectly equivalent statement is that there are certain "strings that are illegal", but this is both trivially true and a clearly exposes the fallacy at play here.
A written death threat is still a death treat no matter the string encoding you use, so while yes it is an "illegal string", this is just incidental: its the threat that the string encodes that is really breaking the law.
Not really disagreeing with you, but taking it even further:
A "written death threat" is not "illegal". I've seen plenty of such threats written on the boundaries of military sites or even electricity substations. Delivering death threats to a person may constitute illegal harassment, but so might delivering love letters. It all depends on the circumstances.
A warning is not the same thing as the sort of threat you get in trouble for. If the circumstances are 'conflating the meaning of terms' then yes, a lot of things - practically everything - can end up 'illegal'.
Everything is a number. When you think of an illegal media, such as child porn, it is in fact a gigantic number that your computer parses and turns into an array of frames which is an array of Red, Green, Blue values. Since everything can be encoded into numbers, and some things are illegal, trivially some numbers are illegal (in certain contexts).
A perfectly equivalent statement is that there are certain "strings that are illegal", but this is both trivially true and a clearly exposes the fallacy at play here.
A written death threat is still a death treat no matter the string encoding you use, so while yes it is an "illegal string", this is just incidental: its the threat that the string encodes that is really breaking the law.