SSMS 17.9 on the left has Database Diagrams at the top.
SSMS 18.0 does not. Database Diagrams are simply gone. Hallelujah! For over a decade, people have repeatedly cursed SSMS as they’ve accidentally clicked on the very top item and tried to expand it. One of the least-used SSMS features had one of the top billings, and generated more swear words than database diagrams.
The good news continues when you right-click on a server, click Properties, and click Processors.
The comments show that not everyone is happy about this, but I do think it’s for the best—the database diagram tool hadn’t been updated in a long time and is missing many features that an ER tool needs. I’d rather use Visio (or a better tool).