Joomla! is a free and open-source content management system (CMS) for publishing content on websites. Joomla 3.0 shipped in 2012 and nine years is a very long time indeed in Internet years, but Joomla 4.0 is here, now.
Joomla 4 is ready for production. I repeat with emphasis. Joomla 4 is ready for production. You can find out all about Joomla's benefits over at joomla.org/4/#benefits.
This IS a Joomla 4 site. At the time of writing it is being updated regularly to the latest releases from the default channel although it has been through all the beta and relese candidate releases.
If you want to you can get involved over at developer.joomla.com