What do you mean by "the reading view"?
Every app publisher decides on their own layout.
Are you talking about Safari Reader Mode? Some particular app you do "reading" in?
Post a picture, no idea what "bar" you are talking about.
Apple now defines a "safe area":
https://developer.apple.com/design/...ines/ios/visual-design/adaptivity-and-layout/
and UI guidelines dictate content should be shown only in the safe area. Background and decoration/flourishes can go outside the safe area. "black bars" will appear on apps that have not been updated. This is imposed by iOS if an app was built for a target version of iOS < 11.
Target version is the HIGHEST version of iOS the app was built for. iOS engages in "adaptive behavior" when running an app on an iOS version > its target version. This is because the developer could not have possibly guessed what Apple will do in the future, and it is desirable for older apps to continue to work on newer versions of iOS with the least-awful side effects.
Frankly, if an app hasn't been updated since iOS 10, you probably should be looking for a replacement app.
For hybrid apps that use a webview, as well as for websites, the safe area is particularly easy to deal with - the webview/browser defines CSS variables for the top/bottom/left/right boundaries.
Notice the pink margin areas in the page I linked above. An older app that has not been updated will probably have a margin to avoid having text get too close to the edges. And so looks fine on an older iOS. But if built for < iOS 11, iOS will add a "black bar" margin. Now you have a nasty black bar, AND still the original margin inside the black bar, and so less space for text.
Developer needs to update the app!
If you are seeing any of this with Safari Reader Mode, then that's on Apple.