I have no idea what would happen if I, for example, downloaded V 3.0.1 of Onyx, and tried to use it, given that I am now running El Capitan. I don't want to try!
Whenever I have upgraded to a major OS (e.g. from Yosemite to El Cap), the old version of OnyX has refused to open, saying that it is not compatible. I assume that the same would happen if you downloaded an older version.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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