Unfortunately, as of this past summer, new plugin developers like me could only use the new way (UXP) with one of Adobe’s desktop apps: Adobe XD. (Internally, Adobe is already using UXP to add new functionality to many of their apps, and it’s in private beta for some Photoshop plugin developers.)
As a result, I created a plugin for Adobe XD. It’s another maze-generator app, but it has more functionality than the one I created for the web. I wrote some docs for it using SnazzyDocs. I submitted my maze-generator plugin to Adobe and they accepted it into the official list of Adobe XD plugins.
Update in January 2021: The maze-generator plugin for Adobe XD has been retired.
Overall, the development experience was quite nice (including help from some nice folks in the Adobe XD developer forums), but I found the vector graphics capabilities of Adobe XD to be quite limited. I guess they will stay that way because Adobe’s flagship vector graphics software is Adobe Illustrator, not XD. That puts XD in a strange position, because its main competitors, Sketch and Figma, have much more vector graphics functionality.
Moreover, not all of XD’s functionality is accessible to plugin developers, or at least not yet.
As a result, I’m no longer as interested in developing plugins for XD.
I’m most interested in developing UXP plugins for Adobe Illustrator (because it has the largest user base), but it seems that won’t be an option until 2021 or later. In the meantime, I can develop plugins for Figma, Sketch (if I get a Mac) or Photoshop (once UXP-for-Photoshop comes out of beta sometime this fall).