Far more likely to me is that bringing libc++ up to C++20 standards isn't important for google3. Carbon is a O(10y) project, not a O(3y) project.

  * absl strings provides the same compile-time guarantees that <format> does
  * absl date/time libs have integrations with the proto time types
  * google3 has many threading & synchronization primitives that it prefers over the STL types (all of which already has integrations with fleet-wide performance monitoring and other toolchain customizations, e.g. deadlock detection), so there isn't much reason to move the threading libraries forward