As it was posted at June 11, 2019, to announce that SQL Server Management Studio (SSMS) 18.1 is now generally available, the Database diagrams is back in SSMS 18.1 along side SSBDIAGNOSE.EXE and Integration Services (SSIS)
Original Answer:
The feature is deprecated, you can visit SQL Server Management Studio – Changelog (SSMS) page and read the Deprecated Features in SSMS 18.0.
Deprecated Features
The following features are no longer available in SSMS:
- T-SQL Debugger
- Database Diagrams
- …
Those features are available in earlier versions, you can use one of them a long side with SSMS 18.0