Earlier quoted context omitted.
Any time I’ve worked with somebody who had to implement deep links, I’ve always heard them continuously complain about how ridiculous it is to get working.
Deep links are only hard because the base concept is semantic organization of your content, so you can deep link to one thing in an organized way. If your app isn't structured to present content like this, if it has many deep workflows or ordered stages of screens/states, then yeah it's gonna be hard to deep link into page 5 of the 11 page workflow you built, that also requires authentication and a bunch of data load…
I once had to implement deep links for a React Native app and, while I’m by no means a great mobile dev there were plenty of thorny bits. For example, I think iOS doesn’t open a deep link in app if you type the url directly into the address bar. It will only open in the app if the user taps a link. Another complexity is making sure a deep link opens in the app for app users and opens the App Store for other users.
iOS also has two different systems for deep linking, custom protocols (eg myapp://) and universal links (a server side manifest that you register with Apple). These each have their pros and cons.
To be sure, those should be solvable problems for a company with even a fraction of Reddit’s resources.