Very basic superscript and subscript buttons for Redactor 9.

redactor_buttons.js

RedactorPlugins.supersub = {
  init: function () {
    this.buttonAdd('superscript', 'Superscript', this.executeSuperscript);
    this.buttonAdd('subscript',   'Subscript',   this.executeSubscript);
  },
  
  executeSuperscript: function () {
    this.execCommand('superscript');
  },
  
  executeSubscript: function () {
    this.execCommand('subscript');
  }
};

$('textarea').redactor({
  plugins: ['supersub']
});