Apple makes all relevant decisions on language design and direction. Swift is Apple's proprietary language in the same sense as C# is Microsoft's.
But it is not proprietary.
By that definition, C#, Go, Java are all proprietary.
By that definition, C#, Go, Java are all proprietary.
While it is heavily funded and used by Apple, I would never call it "proprietary". It's open source.
https://swift.org/about/#swiftorg-and-open-source