These tags should not be synonymised.
I will double check the objective-c-runtime-tagged questions and be certain that they are tagged correctly, but synonymising these tags would actually be quite harmful.
From the Objective-C runtime wiki:
The Objective-C runtime is a runtime support library provided with an implementation of the Objective-C language. Its API allows dynamically creating and configuring classes at runtime, as well as introspecting existing classes, methods, properties, and method implementations.
Very few Objective-C development tasks require interacting with the
Introspecting the runtime environment, for example to produce a live class browser or graph the relationships between classes or
objects at runtime.
Creating a proxy object to interact with the message forwarding machinery.
Dynamically creating classes and methods at runtime, for example, to support a scripting interface.
Optimization hacks for repeated message sends, such as hoisting the method lookup out of the loop and calling the implementing function
Writing an application without using Objective-C, such as C or C++
That all of the objective-c-runtime questions are also tagged objective-c is only by chance.
As the tag wiki comments, we might need to tap into the Objective-C runtime when we're writing a C or C++ application. I'm not sure exactly why in those languages...
However, this answer of mine on Stack Overflow shows a case of using the Objective-C runtime with Swift. If I were to turn the code from that answer into a Code Review question, I'd tag it with swiftobjective-c-runtimeiosuikit probably.