Release 2.4 (202X-XX-XX)

It is now an error to modify the plugin-files setting via a command-line flag that appears after the first non-flag argument to any command, including a subcommand to nix. For example, nix-instantiate default.nix --plugin-files "" must now become nix-instantiate --plugin-files "" default.nix.
Plugins that add new nix subcommands are now actually respected.