Objective-C is a C superset, while Objective-C++ is a C++ superset, where C++ is a mostly C superset. You're not really buying anything using Objective-C++ by adding another form of OOP, except perhaps for interop with C++.
It’s the interop that’s a big deal. The platform is Obj-C (or was, now it’s Swift), most useful non-iOS-specific libraries are C/C++, being able to use Obj-C++ to glue everything together is really convenient. Vastly easier than e.g. JNI on Android.
There it is. True, it's very painful to wrap C++ code into C libraries (or requiring C calling conventions) where C++ isn't allowed.
Separately, I'm wondering if anyone uses Kotlin Multiplatform and how well that works for iOS development.