We were considering using SharedArrayBuffer for certain optimisations in our add-in. However, it seems that strict cross-origin isolation is required for that to work. https://web.dev/coop-coep/ ...
crossOriginIsolated also needs to be supported in Excel Desktop for Mac.
From my initial testing of an add-in with proper headers set, globalThis.crossOriginIsolated returns false.
Any add-in doing complicated logic needs background workers, and SharedArrayBuffer is the best solution for communication with background workers.