> you can't expect the old C code to follow the rules of the new versions of the language
Well, to be pedantic, the entire point of the C standard, and the standard body, is that you should expect it to work, as long as you're working within the standard!
Not really, no. Newer versions of standard can (and do, although rarely, I have give it to C standard committee) introduce incompatibilities with earlier versions of standard. E.g. at one point the standard explicitly allowed to #undef "bool", "true", and "false" (and to redefine them later) but IIRC this has been deprecated and removed.
In any case: blindly switching what is essentially a typedef-ed int into _Bool has no business working as expected, since _Bool is a rather quirky type.
I'm using the dictionary definition of expect here, which is compatible with what you're saying.