One of the developers here. Sketch.systems is about 5 years old and we haven't actively worked on since its release––but it's still used fairly regularly by folks and does the job it set out to do! If anything ever seems broken, reach out (our Twitter accounts are in the website footer) and we'll definitely take a look.
I try to mistake actively developed on for needs active development.
Some of the tools I like haven’t significantly changed for me in years. There sometimes is a “good enough” to software that I don’t need a developer to work on it endlessly for the rest of their life.
But I will say that my interest in this, it was dashed after your comment. This looks like a tool is complex enough to probably not be perfect.