Why is my Toggle field not showing in Gutenberg?

You forgot to declare the attributes:

registerPlugin( 'page-layout-plugin', {
    icon: 'smiley',
    attributes: {
      'showSidebarAttribute': {'type': 'boolean', 'default': true}, // <-- and maybe others
    },
    render: ( props ) => {

You can drop the quotes of the properties, of course (they are optional), I keep them because the equivalent statement is copied over from the PHP part where the quotes are necessary:

 'showSidebarAttribute' => ['type' => 'boolean', 'default' => true],

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