If you don't invest in companies that don't release hardware unit sales, your options are rather limited since the following companies don't provide that data either:
Why would it not be the case? The important thing to report to investors is the financials. The last thing you want to do as a company is advertise to your competitors exactly which of your products are failing and which are succeeding. Also, it would be insane if companies had to report "Unit sales" do they have to report iPhone 7/8/9/X? Or 7/8/9/X + different colours? How about X vs X+? Do you need to know iPhone X+ White 64GB sales?
They stopped doing that multiple quarters ago because it wasn't growing and they want investors to focus on other parts of the business and not just the iPhone.
If you believe the strategy laid out in the parent comment to yours, then it isn't a red flag at all. For the last 10 years, the number 1 thing that contributed to Apple's growth was the iPhone. They don't believe that will be the case for the next 10 years.
I think they should release those numbers for investor clarity, but I don't think it's a red flag.
They stopped detailing their exact Mac and iPad sales too, so I dont see the problem.
I realize iPhone is an amazing amount of their revenue, but they are transitioning to a services and accessory business built around iPhone. It’s less important exactly what model iPhone you have, in my opinion.
Dell, HP, Microsoft, Amazon, Google, Samsung, LG... etc etc
Red Flags everywhere