I can force the installation with:
SYSTEM_VERSION_COMPAT=0 pip install tensorflow-macos==2.7.0 tensorflow-metal==0.3.0 
SYSTEM_VERSION_COMPAT=0 pip install tensorflow-federated==0.19.0

and got an error:
ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
tensorflow-macos 2.7.0 requires tensorflow-estimator<2.8,~=2.7.0rc0, but you have tensorflow-estimator 2.5.0 which is incompatible.

It appears to me that my tff examples will still work. But it would be really nice to have an update on tensorflow-federated with dependency on tensorflow-estimator>=2.7.0