Since the last update I can not start taskpaper on os x 10.10.5 anymore… 
Now that 10.12 is coming right around the corner I expect I’ll just support that and 10.11. Supporting 10.10 is difficult because it means I need to maintain a separate computer for it (and soon that computer will be for testing 10.11, as I upgrade to 10.12).
That is sad news. I stopped following the 3.5 Preview Releases because they only worked on OS X 10.11. But back then this was only true for the Preview releases and the final 3.5 release would support OS X 10.10. All the more was I disappointed to see, that the final 3.5 release dropped support for OS X 10.10 along the way.
I understand your reasoning regarding hardware to test on and the adoption rate of new MacOS releases will probably support your decision. Nevertheless, using TaskPaper daily I have a hard time being stuck on version 3.3.2 and missing out on all the great new features. 