Special:Mute can be accessed directly, but currently doing so results in an error message like "The username requested could not be found. " (specialmute-error-invalid-user). This is not as intuitive as showing a form for looking up a user to block/unblock.

Coincidentally, that is basically how e.g. SocialProfile's Special:RemoveAvatar page behaves when you're a privileged user (someone with the avatarremove user right, i.e. someone who can delete other people's avatars and not just your own): if the page is accessed without a username supplied, the user is presented with a (very basic) form for looking up a user whose avatar to remove.