Thanks for your answer, Joseph. I was afraid this might be the case. I'm wondering whether Apple will add the dispatchMessage functionality to iOS. Do you know where I can request this to Apple?

I'm also very keen to know if there will be a version of SFSafariApplication.dispatchMessage coming to iOS soon. Would we be able to request this somewhere? The beginRequest(with:) function is also something I need help with, as the messaging is working, but I need to be able to do something with the data being returned. i can't seem to work out how to write this data to the group container. Any help would be fantastic! I asked a similar question here – SFSafariApplication.dispatchMessage for iOS Safari Web Extension? – and the reply mentions the functionality isn't explicitly supported. Would be good to know if this is set to change or stay unsupported.