To Reproduce:

1. Create a new project.
2. In Package Manager disable Physics2D package and turn off engine stripping in player settings.
3. Build and run on an iOS device.
4. Observe that the player crashes with:

...
* frame #0: 0x05a806d0 ...`::Initialize() at PhysicsManager2D.cpp:437 [opt]
frame #1: 0x051269bc ...`::CallInitializeTypes() at TypeManager.cpp:136 [opt]
frame #2: 0x0564e564 ...`::InitializeEngineNoGraphics() at SaveAndLoadHelper.cpp:236 [opt]
frame #3: 0x0566acb4 ...`::PlayerInitEngineNoGraphics() at Player.cpp:384 [opt]
frame #4: 0x05e99e04 ...`::UnityInitApplicationNoGraphics() at LibEntryPoint.mm:188 [opt]
frame #5: 0x000ba042 ...`::-[UnityAppController application:didFinishLaunchingWithOptions:](self=0x16eb4f20,
...

the module is never initialised from Physics2DModuleRegistration.cpp, but we still attempt to initialise PhysicsManager2D.cpp from Physics2DSettings. This does not seem to happen with other builtin packages (like Physics etc.)

Reproduced with:
2018.2.8f1, 2018.3.0b2, 2019.1.0a1
iPhone 7+ iOS 11.1.1, iPod Touch 6 iOS 10.2.1

Not reproducible with:
Android, Desktop Standalone

Note:
Full call stack attached in the edit below