It's hidden. The Start-up folder still exists and functions as it always did. It's just not shown on the Start menu anymore (not unless you use something like Classic Shell, that is).
Originally Posted by martian
In fact (as before) there are actually two Start menus, one for All Users and a personal one for each user. Microsoft would seem to prefer install programs to add an entry in the appropriate Run registry key (again, there are two). The start-up folder still exists for (presumably) backward compatibility.
As it has been since Start was first seen in Windows 95, the two Start menus (all users and personal) are merged into one and shown as a single Start menu. For Windows 10 their start-up folders are here:
All users: C:\ProgramData\Microsoft\Windows\Start Menu\Programs\StartUp
Per User: C:\Users\<username>\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup