Let's take this example: https://webrtc.github.io/samples/src/content/devices/input-output/
If you load that page with Desktop Chrome, Desktop Safari, Chrome from Android, Chrome from iPhone, Safari from iPhone, ... anywhere ... it works perfectly.
But, if you send that URL through a personal message in LinkedIn (I tested with this app, but I guess it will happen in all in-app browsers in iOS), and you open the link within the iPhone LinkedIn app (this is the user_agent: "Mozilla/5.0 (iPhone; CPU iPhone OS 15_1_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148 [LinkedInApp]"), it does not work.
After investigating, I've checked that the problem is the "audio" in constraints.
If I put the constraints with { video:true, audio: false } it works.
For example, here is an example working (again, from iPhone LinkedIn app), because it does not use "audio": https://webrtc.github.io/samples/src/content/getusermedia/gum/
But if you add "audio: true" to this last example, it does not work, it goes to the handleError function with a message: "The request is not allowed by the user agent or the platform in the current context, possibly because the user denied permission."
What is happening with this browser and WebRTC?
I see no clues or info in Internet, and I think it's happening since the iOS 15.1, because it was working perfectly in 14_5_1, 14_7_1, 14_8, 15_0_1