I made a few simple Mac apps in Objective-C during the pandemic in 2020, and I was struck by how hard it was to find tutorials and examples on how to do anything in Objective-C. It's possible I was looking in all the wrong places, but it seems like everyone assumes you're using Swift at this point. I would advise against learning Mac development with Objective-C except to people who have a specific interest in it or need for it. I also think most people accustomed to newer programming languages would probably find it arcane in some areas and tedious in many others (although I personally like it for what it is and don't regret the time I spent with it, I definitely felt like I was making things harder on myself by using it).
- Performance: Sometimes the automatic conversion between Objective C and Swift types causes performance issues. Using Objective-C for some hot paths can be a lot faster. Sometimes just using NSString instead of String in Swift can also do the trick.
- Legacy code: For most projects, there is no point in converting 100s of legacy Objective C classes to Swift. Just leave the old parts in Objective C, and call them from Swift. Rewriting just risks breaking stuff.
- Interface with C/C++ code. Swift has some nice bridging that make calling C code easy (such as automatic conversion of swift strings), but some APIs like socket() are close to unusable in Swift, because they require complicated casts that are impossible to understand. Easier to write those parts in Objective C.
