You can download a Free Demo of Purity directly from the Sonic Cat website to test its compatibility with your macOS version.
Luxonix previously released LFX-1310 , a free multi-effects plugin based on the Purity engine. While it isn’t the full instrument, it offers a piece of the Purity sound for free. Purity Vst Free Download Mac
Despite its age, Purity is often preferred over modern "heavyweight" plugins like Omnisphere or Serum for several reasons: You can download a Free Demo of Purity