It is for me (nix flake update a few days ago)
╰─>$ ls -lha $(readlink $(readlink $(which mu)))
-r-xr-xr-x 2 root root 1.6M Dec 31 1969 /nix/store/89jrndbn0zcc602is7f2kipwyihxdf56-mu-1.10.8/bin/mu
╰─>$ ls /nix/store/89jrndbn0zcc602is7f2kipwyihxdf56-mu-1.10.8/bin
mu
I have both mu in extraPackages input to the doom constructor/override (with nix-doom-emacs) and programs.mu.enable = true; in home-manager. It may be because my exwm install doesn’t expose doom as an app, so I have a lot of stuff I can try.
Update (fixed): the thing I was looking for was adding both pkgs.mu and pkgs.mu.mu4e (as extraPackages to doom), rebuilding then rebooting. I didn’t disable the programs.mu.enable in home-manager, I’m not sure what it’s doing right now.