Hi Riccardo,
That will work, but you'll lose new functionality and bug fixes implemented in the latest updates. More importantly, the next version of PixInsight won't work on anything older than OS X 10.10; perhaps 10.11. That's how the world of Apple and OS X is: We can only provide compatibility with at most the two most recent versions of the macOS operating system, and sometimes even that is a big challenge for me.