It's true Adobe AIR is derived from Flash Player, but it keeps improving. It can be used for developing rich-interface applications (RIAs) and games. Although it's legacy, it's a pretty productive SDK. It's now maintained by Samsung Harman:
If I understand correctly, it's gonna get WebAssembly support futurely. I've posted about this possibility of using AIR from Rust in another community: Adobe AIR SDK for Rust?
Note AIR doesn't come with UI capabilities at all; a library is built on top of the display list. There are frameworks out there such as Feathers UI (which is currently a priority for the Haxe language right now).
So, essentially it's an alternative to the browser platform. In some ways it's more efficient. It's also lightweighter. I'd personally build my own SDK on top of AIR.