CPS[1] is where all the effort is currently going for a C++ packaging standard, CMake shipped it in 4.3 and Meson is working on it. Pkgconf maintainer said they have vague plans to support at some point.

There's no current effort to standardize what a package registry is or how build frontends and backends communicate (a la PEP 517/518), though its a constant topic of discussion.

[1]: https://github.com/cps-org/cps