adding wp-cli commands to plugin: how to document to users?

There’s the wp-cli tag that is being used in WordPress plugin directory, but it is not super popupar (less than 500 plugins seem to use it). wpcli is not even close with less than 20.

In general you’ve listed all the other ways that you can inform your users unintrusively about the presence of a WP-CLI command with your plugin.

You could, of course, add an article in your plugin’s website, the extra documentation that you might have, in the newsletter for the plugin, etc., but this is all extra efforts for someone who’s making money out of it or insists on making it a well-known fact by everyone.