Backwards compatibility with what? Rust has always has had enum also for this exact case, to avoid encoding sentinels as strings or pointers.

im quite positive the only rust programmer who would do this is a C programmer who doesn't understand rust

because rust has features that makes doing it pointless.

in theory could someone do it? yeah, just like in theory someone could program a calculator to randomly rm -rf ~ when pressing +.

but in practice would anyone do it? no.