I don't have data, but there's no way that the Mac developer ecosystem is worse off than it was in the 00s. There's significantly more Mac users now, and there's orders of magnitude more developers with experience developing for Apple platforms.
Maybe most devs won't venture outside of iOS to try Mac development, but 10 years ago the few long-time Mac devs were (by necessity) putting their Mac projects on hold to work on iPhone apps.
> because Apple is strongly implying that it's deprecated
Lots of iOS developers are writing blog posts claiming things like this, but every single Apple app on the Mac is written using AppKit. You can't deprecate the technology behind your entire platform.
Maybe 15 years from now SwiftUI will have replaced AppKit as the dominant way to write UI code on the Mac, but generations of apps will be born and die between now and then.
I don't have hard data either. But ask any old Mac developer on Twitter whether they're doing better now than ten years ago, and I bet a majority would disagree.
For one thing, the rise of mobile app stores has destroyed the perceived value of software. A $50 app now seems very expensive to most, whereas it was mid-priced back in 2008. Yet the Mac App Store has failed to bring in the mass audience that would compensate for the lower unit prices.