This is indeed an unintended removal. It only affected SQLEXPRESS.
Thanks for the reporting it.
We’ll fix it in a future release of SSMS 18.×.Bruce L. Scheffler commented
It seems that another function "copy database" is also missing in the new version. However the disabled/missing function only happen if you are trying to use the new version with sql express. If you have an actual sql server setup it is there and is enabled.