Flake template isn't working throwing llvmPackage issue

$ nix flake init -t 'github:mitchellh/zig-overlay#compiler-dev'

wrote: /home/linuxmobile/Dev/ziglings/.envrc
wrote: /home/linuxmobile/Dev/ziglings/flake.lock
wrote: /home/linuxmobile/Dev/ziglings/flake.nix
wrote: /home/linuxmobile/Dev/ziglings/shell.nix
nix-direnv: /home/linuxmobile/Dev/ziglings/.envrc                                                                                  
/home/linuxmobile/.config/direnv/direnvrc
/home/linuxmobile/Dev/ziglings/shell.nix
error:
       … while evaluating an expression to select 'drvPath' on it
         at «internal»:1:552:
       … while evaluating strict
         at «internal»:1:552:
       (stack trace truncated; use '--show-trace' to show the full trace)

       error: undefined variable 'llvmPackages_21'
       at /nix/store/v4wkivmqsf4h743pb4d85rhjm5iyc41k-ziglings/flake.nix:36:22:
           35|             ]
           36|             ++ (with llvmPackages_21; [
             |                      ^
           37|               clang

       note: trace involved the following derivations:
       derivation 'nix-shell'

Replacing line 36 make it work:

# flake.nix
-            ++ (with llvmPackages_21; [
+            ++ (with llvmPackages; [

Start with flake.nix around line 36 and reproduce the failure using the reported nix flake init -t 'github:mitchellh/zig-overlay#compiler-dev' command. Check why llvmPackages_21 is unavailable in the generated environment; done means the compiler-dev template evaluates successfully without the undefined-variable error.