Work continues on the Wii U emulator Cemu to bring it over to Linux, and with that gain Steam Deck support. The developers still plan to open source it too, with their roadmap indicating it's still scheduled to happen this year.
For the Linux port, the developers put up an Imgur post going over the current status. An important milestone has been hit recently, with the project being able to be compiled on Linux "without errors". After that, they said it "didn't take long until we were able to boot the first few games" with a screenshot shared of it running on a Steam Deck:
Plenty more work is still to be done though, so don't expect it too soon. Lots of issues remain from UI problems to porting over various other parts like networking and sound. From what they said we're looking at "1-3 months" before it will be ready to release in public.
They also showed off that it can do OpenGL and H264 video decoding in the port, along with a shot of some of the broken UI. So a ways to go yet but it's coming along nicely it seems.