The introduction of the Block Editor killed all plugins which offered publishing conditions, such as minimum word counts, featured image requirements etc.

But the Block Editor did introduce the pre-publish checks:

Beautiful. How can we disable the Publish button until a set amount of conditions have been fulfilled?

Examples of four (very) different conditions:

1. Minimum word count (example: 500 words)
2. Min/max tags (example: 3-5 tags)
3. Min category (that isn't uncategorized)
4. Featured image is assigned

What we have so far

As expected, the documentation is non-existent. But leads are scattered across the web.

In core/editor, we can use .lockPostSaving() to disabled the Publish button, and unlock it via .unlockPostSaving().

We can add a panel to the pre-publish screen via PluginPrePublishPanel. Example (by MadMaardigan):

var PluginPrePublishPanel = wp.editPost.PluginPrePublishPanel;
var registerPlugin = wp.plugins.registerPlugin;

function Component() {
    // lock post saving
    wp.data.dispatch('core/editor').lockPostSaving()

    // unlock post saving
    // wp.data.dispatch('core/editor').unlockPostSaving()

    return wp.element.createElement(
        PluginPrePublishPanel,
        {   
            className: 'my-plugin-publish-panel',
            title: 'Panel title',
            initialOpen: true,
        },  
        'Panel content'
    );  
}

registerPlugin( 'my-plugin', {
  render: Component,
});

It works:

We also have great discussions on GitHub: #7020, #7426, #13413, #15568, #10649...