I'm afraid the letter spread some misinformation: The meson RFC has in fact not been approved: https://github.com/NixOS/rfcs/pull/132
For a discussion on this, see https://github.com/KFearsoff/nix-drama-explained/issues/1
For a discussion on this, see https://github.com/KFearsoff/nix-drama-explained/issues/1
> Single-handedly blocking Meson, which has a passed RFC, for five years because he doesn’t know meson but knows his own build system
I don't know about exact timelines, but the letter is definitely wrong about this part.