It looks like Blizzard had to move to a newer version of XCode that can no longer target anything prior to Big Sur (macOS 11). World of Warcraft had that issue when it transitioned to being built with a version of XCode that could target ARM64 (M-Series Macs).
Navigate to your Diablo 3 folder in the Finder. If there is a stop sign overlayed on top of the Diablo 3 launcher and app, that means you need to be on a newer OS in order to launch the game. This is something end users cannot override.