Modern Objective-C with ARC is fairly painless, especially compared to the Objective-C of yore. And if you are doing macOS-specific work for a C/C++ codebase, it's still simpler to integrate than Swift is.

Method swizzling is also an important tool for any aspiring macOS developer, unavailable/onerously painful in Swift.

All in all, I don't think there's anything wrong with continuing to write new things in modern Obj-C as long as you're conscious of the tradeoffs and interoperability is a concern.