“Plugin closed” means that the plugin you are reading about is no longer being maintained or updated.
It is free and easy to create a new WordPress plugin on wordpress.org, which has over the years resulted in thousands of plugins that are no longer actively maintained. Many are still usable, but there are many plugins that are no longer usable and contains verified security issues.
When security researchers discover an issue with a plugin on wordpress.org, the WordPress team will still list the plugin on the website for historical reasons, but you are no longer able to download the plugin.
Any plugin that has been closed should be removed immediately from your website. If the plugin provides a critical function there should be alternative plugins available. It is not recommended to keep a plugin active that has been closed.
