A formal technical specification (TS) extension is already being drafted: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3582.pdf That's a step up from the usual proposals. I'm not sure what criteria is used to decide whether to first create a TS vs just incorporating a change into the working draft of the next standard.[1] _Defer also seems to be taking the TS route.[2]
Well, 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.
1. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3886.pdf
2. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3928.pdf