This was a bad idea, IMHO. Plugins are not like themes and some plugins won’t even work without [another] plugins. Like some plugin that extends another plugin. How would wp.org expect to have a “Playground” that would load every dependency available to mankind.

Plugins SHOULD ALL have a demo link. why would you build a plugin and then HOPE that people understand how to use it just by reading the readme text you have injected into the WP repo for your plugin page. ALWAYS add a demo link. Usually the last sentence in the description is where I add mine. why not. It can’t hurt and it is much better to create a bunch of sub-domains or sub folders on one of your hosting instances; this way people know you have tested it and that you are supporting it in a very “customer first” manner.