https://www.bleepingcomputer.com/news/microsoft/microsoft-starts-notifying-users-if-windows-10-2004-is-blocked/

Microsoft has begun to notify users via Windows Update if they are currently blocked from upgrading to Windows 10 2004 due to a compatibility hold.

With the May 2020 Update release, Microsoft placed nine compatibility holds on devices that prevent them from upgrading to Windows 10 2004 if they are using specific drivers, applications, or Windows 10 features.