Your note says SS 2.9.28 runs under macOS 10.12.4, but I’m running 10.10.5 (MacPro 4,1) and cannot upgrade to the more recently released MacOS. Should I upgrade SS to 2.9.28 or not?
Yes, quoting the blog post:
SpamSieve 2.9.28 works with Mac OS X 10.6 through macOS 10.12
I recommend that everyone using those OS versions update.