There's a roadmap.md on github showing MS's intentions for WPF -- first and foremost, porting to .Net Core. So there is, at least, active (and I'd say significant) development. But...
Not trying to sound facetious, but does WPF need any other significant work? It's a UI framework for Windows desktop apps (narrow and specific scope), it's stable and "just works", it's well-documented, it's officially supported, etc. Does there need to be anything else, or are we looking at it from the POV of more "modern" UI frameworks, where no active churn == languishing on the wayside?
Also, the relationship between UWP XAML and WPF XAML is a lot closer than (for example) Win16 versus Win32 or MFC versus WinForms. Even if WPF is "just in maintenance mode", UWP is seeing active development. Some of UWP XAML additions have even back-ported to WPF, and "XAML Islands" even mean support for putting UWP XAML directly inside WPF XAML.
Not trying to sound facetious, but does WPF need any other significant work? It's a UI framework for Windows desktop apps (narrow and specific scope), it's stable and "just works", it's well-documented, it's officially supported, etc. Does there need to be anything else, or are we looking at it from the POV of more "modern" UI frameworks, where no active churn == languishing on the wayside?