> nothing that would prevent me from writing "C with classes" using the modern version.

Or indeed "C without classes", just with some extra type-checking.

And of course K&R used Stroustrup's C++ compiler to build and test the code for TCPL 2nd Ed.

Languages are both read and written, restrictions like OP is pining for are fundamentally for reading. As such, it is not terribly helpful that they can opt in to restrictions when writing.