Roon Server is part of the “Roon” install. For Mac devices, it can be found at Roon.dmg
This has been the case since the release of 2.0.25 - see Roon 2.0.25 is live.
The Roon installer will now install two applications: Roon and Roon Server. To use Roon, you will need to start Roon Server on a device (and probably configure it to start on power on).