A2007 still supports the A2000 and A2002/3 format .mdb files, and Workgroup
Security still works in those formats... so it's not altogether gone. What you
can't do is apply workgroup security to one of the newer .accdb format
databases.
One reason they took it away is that it provides a false sense of security.
It's pretty easy to find "cracks" on the Web, which can break into a database
with workgroup security: "Locks only keep out honest people". There is
probably NO file-based (like Access) system which can be made really secure.
A server-based system such as SQL/Server can be broken, too, but it's a whole
LOT more work for the criminal - all but prohibitive if the SQL security is
set up correctly and people take appropriate precautions with their passwords
and logons.
In addition, workgroup security was just *so* complicated that it was often
applied incorrectly, causing more problems than it solved.