According to Adobe's website, support for Flash will be dropped this year (2020). They advise any business or website that is currently using Flash to migrate over to a more universal platform, such as HTML5.
xat's HTML5 version of the chat and website are currently under development. By default, all xat groups use the latest HTML5 version.
Yes, for now you can manually add "?old" at the end of a group link to use the Flash chat. For example, "xat.com/Help?old". If you wish to use the Flash version on all groups by default, you can activate this setting on the login page.