Sibelius 8.6 will not work fully correctly on any macOS beyond Mojave. It will launch and apparently run under Catalina, but certain things will be broken (anything to do with video, anything to do with Avid Application Manager). Under Big Sur, it gets worse: there is difficulty entering any text.
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Sibelius 2021.2/7.5.1/7.1.3/6.2/5.2.5, PhotoScore Ult 2020.1, Windows 10 64-bit 16GB. Desktop, and Microsoft Surface Book.