A couple weeks before iOS 15 was released, I had developed a PHP app that sends generate tokens to the mobile app. The link looked like this my-app://?Token. The button was clickable and the app did open. After the update, the button is no longer clickable. I checked on Outlook mobile app, Office 365, and Gmail web browser with an iPad, iPhone, and Xcode simulated devices.
To troubleshoot:
I just displayed the link in the email. The link does not display as a clickable link just text.
I wrapped the link in http://my-app://?Token and of course the link was now clickable, but this does not open the app.
Tried on iPad, iPhone and Macbook. The Macbook link is clickable. This makes me believe that is a mobile iOS issue.