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

Is that even the same issue?

It is the root cause of some issue with Trello that appeared with Safari 14.1, but perhaps not the same issue causing other sites to break; if so, perhaps there are multiple bugs with the update.