I encounter the same issue which for me breaks Microsoft's documentation, e.g. https://docs.microsoft.com/en-us/dotnet/api/system.io.memory...

I get (macOS 10.14.6, Safari 14.1):

  > new Intl.NumberFormat('en-US', {notation: 'compact'}).format(1825602)
  < TypeError: Failed to initialize NumberFormat since used feature is not supported in the linked ICU version