Is Scapple abandonware, or the next best thing?

Yes

Umm … the info in the sidebar says Mac, and this is the Mac forum, but you say here you’re a Windows user!

But to put some flesh on the bone @Lunk has thrown you, Scapple for Windows is coded by LAP and Tiho_D who are currently deep in trying to get Scrivener v. 3 for Windows ready for launch. I doubt if they have any time and energy at the moment to turn their attention to Scapple.

Equally, Scapple for Mac is developed by KB, the ultimate arbiter, who also single-handedly develops Scrivener for both MacOS and iOS. While I can have no idea what he is working on at the moment, he has said in the past that he has new developments for Scapple that he would get round to when he could find time. In the context of all that, there is of course Catalina 10.15 for Mac and iOS 13 due out from Apple sometime soonish, for which he must be working on any compatibility issues.

Mark