It's also widely implemented in practice. For example, Instagram allows copying links to posts, and if you can view an Instragram link in the browser, it opens in the app.
ios and mac got url schemes. Sure not all developers implement it correctly. But not all sites implement a good url structure either (keeping state etc in url for sharing)
iOS apps absolutely do support deep links and URLs. Custom URL schemes are the older of the two and let iOS launch the app when someone taps on a URL that begins with its protocol (e.g. "bugmunch://orders?id=xxxxxxx"); the newer is Universal Links, which lets you set your web site up to launch your app from the web page if it's installed on your device.
Native apps made with SwiftUI or Jetpack Compose adjust their font size according to user preference in system settings out of the box. They also relayout automatically. Copying text on press is a one-liner in both frameworks, just need to think of it.
Dunno, native app doesn't have URLs nor deep links. I can't link you to a page in my native app that you can click on.
In my iOS HN app I'd have to click "Copy website link" to share a submission. If there were no website, I couldn't share it at all. Same with Reddit.
For most apps, that's a lot to give up when you app is basically just a website without a url bar.