There's some discussion of how C29 defer ended up in a TS at https://thephd.dev/c2y-the-defer-technical-specification-its... .
replyWell, even if that lands on the official standard, that means C29 as probable release year for C2y, plus adoption of the exact form across compilers to be able to rely on it being available.
replyPlus the fact that an implementation that recognizes the new syntax and produces an error whenever it encounters it will be conforming.
replyOn such an implementation, the feature is available but useless.