As of August 2020, Windows 2000, XP and Vista are no longer downloadable via Windows Update.
How?
Windows Update's servers have used SHA-1 and SHA-2 for a while now. As of August 2020, Microsoft has transitioned from SHA-1 to SHA-2 completely, removing SHA-1 support (which 2000, XP, and Vista need).
Why?
Security reasons. SHA-1 is more vulnerable to security risks.
As an alternative to Windows Update, you will need to use this Update Repository:
You can also use Server 2008 updates, which you can find here: https://mega.nz/folder/txxRyLzC#1vBMGzMHiL864f3bl1Rj1w (created by @greenhillmaniac)