Google has changed the default landing page for Google Sites. Now when you visit sites.google.com, instead of taking you to classic Sites, you will be taken to new Sites.
If you want to continue to use or manage your classic Sites, you can still do so by visiting sites.google.com/classic or clicking the button in the lower-left corner of the new Sites page:
This change began showing up in U-M Google on May 20, 2020, but it may take some time to become fully visible to everyone in the domain.
Learn more about new Google Sites: