While at E3, I got a notification on my phone that the Steam Link app on iOS had received an update. I still have a beta of the app on my iPhone from when I tested it prior to its release—a release that ultimately didn't happen because Apple rejected the Steam Link app for reasons that were a shade unclear. Currently, the app is only available for Android devices.
At the time, Valve stated that the Steam Link app, after initial approval by Apple, had been rejected due to 'business conflicts with app guidelines'. Valve wouldn't elaborate on the specifics, but it's becoming a bit more clear since Apple recently made changes to some of those guidelines. It now states that streaming and remote mirroring apps must limit transactions to the desktop host. Purchases can't be made through the streaming app itself.
The recent update to the Steam Link app for iOS reflects this policy change as well. Opening the Steam Link app today on my iPhone, the Steam store is still available to browse but you can't buy anything from it. As you can see in the screenshot above taken from my phone, you're directed to visit the Steam Store on PC in order to make a purchase.
I've reached out to Valve for comment, and I'll update if I hear back. In the meantime, it looks like the 'business conflict' maybe be closer to being resolved.