I've got an Xcode 10.3 project that uses 12.2 and a new Xcode 11.2 project that uses 12.4 and both are missing simulators like the iPhone SE (which can even update to iOS 13!).
If I change the deployment target to 12.0, all the old simulators show up in the dropdown list again but with every target newer than that, only the simulators for the iPhone 8 (Plus), iPhone 11 (Plus) and a couple of iPads are listed.
How do I get them back?
In Xcode 11.x, select Window | Devices and Simulators. In the window that's displayed, click on the Simulators tab. In the lower-left corner, click on "+" and choose a "Device Type". You can leave the Simulator name field blank as it will use the default. You can also leave the OS version as is. Or, you can always choose to load the older OS versions too.
Note: I had previously loaded the iOS 12 versions in Xcode 11, thinking it would only be for one device (I added the simulator for iPhone 8 and iOS 12). But that ended up giving me iOS 12 flavors for every single configured simulator device. I eneded up deleting that later. I have real devices still on iOS 12, so with the simulators in Xcode, I just use the latest OS it provides (iOS 13.x in this case).