There is a technical specification, so hopefully it will be standard C in the next version. And given that gcc and clang already have implementatians (and gcc has had a way to do it for a long time, although the syntax is quite different).

It is not yet a technical specification, just a draft for one, but this will hopefully change this year, and the defer patch has not been merged into GCC yet. So I guess it will become part of C at some point if experience with it is good, but at this time it is an extension.