What does “diagnostics are in flight” mean? Is my iPhone hacked?

Well, Apple will never send a message like this, so you know that wherever this leads.....it will be a scam to try to trick you into buying useless software or worse.....providing personal information to the bad guys.

Delete the message and move on. NEVER click or tap on a link in a message like this.

Hey, just to clarify I just had the same thing. This is an Apple feature according to Apple Support. When it came on my device I believe what triggered it was when I tried to take a screen shot and hit both volume buttons. Let me know if this helps! Have great day!