Using upgrade and remove is counter intuitive. The following example shows what a user will likely presume to work:

$ nix profile install github:cachix/devenv/latest
$ nix profile upgrade github:cachix/devenv/latest
warning: 'github:cachix/devenv/latest' does not match any packages
warning: Use 'nix profile list' to see the current profile.
$ nix profile remove devenv
warning: 'devenv' does not match any packages
warning: Use 'nix profile list' to see the current profile.

• The installable used in install is not applicable to remove nor upgrade.
• The output shows warnings, but are actually errors. Nothing has changed to the profile.
• nix profile list is suggested, but it isn't clear that you need to pick the index from that list.
• The name of the installable (devenv) is not applicable to remove nor upgrade. Ideally the NAME is deferred from the flake patterns (github:OWNER/NAME/VERSION, github:OWNER/REPO/VERSION#NAME, nixpkgs#NAME, github:OWNER/NAME/VERSION#default).