>The two main backers (Apple and Google) seem to have backed out.

Google has not "backed out".

E.g. look at this topic's discussion thread. The current top comment is about [[clang::musttail]]; the implementor of that attribute works for Google. The current second top comment is about the Rust compilation wins from the switch to the New Pass Manager; the NPM was primarily implemented by (and turned on as the default pass manager upstream) by people who work for Google. Google is actually growing our investment in people contributing to core Clang/LLVM.

Source: I am a manager for C++ toolchains (those that target production, Chrome, ChromeOS) at Google.