Static Data Masking does not appear in SSMS 18 preview 6
It was removed from the final version of SSMS 18. Interestingly, no explanation deemed necessary to be given as why it was taken out or will this feature be coming soon.

From SQL Server Management Studio (SSMS) 18.0 released for general availability by Brad Groux:


  
  Removed static data masking (preview) feature – While this feature was available in previous previews, it did not make it into the GA version of SSMS, unfortunately.