WordPress 5.9 is software is under development. You don’t want to run this version on a production site. Instead, it is recommended that you run this on a test site. This will allow you to test out the new version. We will be testing WordPress 5.9 Beta 1 on customer websites where a support agreement with 3 or more hours is in place (testing takes approx 1 hour on an average website).
The current target for the final release of WordPress 5.9 is January 25, 2022, which is just eight weeks away so testing compatibility with your site sooner, rather than later, is essential!