Microsoft has begun to notify users via Windows Update if they are blocked from upgrading to Windows 10 2004 due to a compatibility hold. Microsoft is working with vendors to release updated drivers that resolve any conflicts. Windows 10 users are now reporting that Windows Update is notifying them when their device is under a compatibility holds and is not allowed to update. The holds are due to conflicts that could cause applications not to run correctly, Windows 10 to have Blue Screen of Death (BSOD) crashes, or Windows 10 problems starting.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-starts-notifying-users-if-windows-10-2004-is-blocked/