I'm updating my app icon to match the new style used in Big Sur, and I'm wondering if it's possible to include an alternate icon for use on older macOS versions so that the app will still look at home next to their built-in app icons?
I've had it confirmed in my bug report that Apple will not support this:
Thank you for your feedback, it is noted. Engineering has determined that there are currently no plans to address this issue, because there are no plans to support non-Big Sur icons.
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