The Feedback Assistant iOS app is visible and acts like a normal app on beta builds of iOS, but it’s hidden in release versions of iOS. On release builds the app icon is hidden on the Springboard/Home screen, it doesn’t come up in Spotlight searches, Siri can’t find it, and the process is hidden in the process switcher if you do get it launched.
The magic incantation to make it visible on release builds of iOS is to enter:
in the Safari address bar to make the Feedback Assistant app on iOS always visible (spotlight, app icon, Siri search, process switcher, etc).
or, if you want to leave it hidden and auto-hiding but want to launch Feedback Assistant to file a new feedback enter the following in Safari:
alone to switch to Feedback Assistant without changing visibility or starting a new feedback (because Feedback Assistant is hidden from the process switcher, you need this if you are in Feedback Assistant and want/need to switch to another app but want to come back to Feedback Assistant to finish the feedback you’re editing, for example).
This is somewhat documented by Apple here: https://developer.apple.com/bug-reporting/
Note 1: You’ll need an Apple ID to file feedback reports.
Note 2: You can always go to feedbackassistant.apple.com to access your feedback assistant account without the Feedback Assistant iOS app, but you can’t include sysdiagnose files and it doesn’t have quite the same ease of use as the app.