For the first time since February Microsoft have released an ISO from the Fast Ring of the Windows Insider Programme. The last fast ring build ISO was a 20H1 / Windows 10 May 2020 build but since then the fast ring has changed from being the next version of Windows 10 to an active development of Windows 10. The new ISO released is 19619 from the active development branch of Windows 10 and was released to the fast ring April 29th.
The ISO is designed for Windows Insiders to test clean installations of Windows 10. You can download the ISO from Microsoft’s download page.