I have seen a few people argue that there’s no reason to let ProGuard run on the open-source libraries that they include in their app, because if the source is publicly available, there’s no point of obfuscating, right? Who are you fooling? Here’s an example from one library’s README file:

Their ProGuard rules file is just a big wildcard keep:

I think that’s exactly wrong, for two reasons — shrinking, and obfuscation.

Shrinking

The majority of the shrinking that ProGuard performs is by removing unused code from third-party libraries. You’re not writing a lot of code that you don’t use, but you’re probably not using all of the features in the third-party libraries that you include. If you use a

-keep com.example.** { *; }

-type rule on open source libraries, you won’t get the benefits from ProGuard’s shrinking. None of that unused code will be removed.

Obfuscation

I’m going to show this with an illustration. What does this class do?

The eagle-eyed among you may recognize a couple of clues and have a hunch, but I doubt you know everything that it’s doing. So let’s look at the same class, but without obfuscating the third-party libraries.

It’s a lot easier to figure out what this code does when the libraries aren’t obfuscated. Simply put, obfuscating the open-source code allows for better obfuscation of your own your code.
(I mention ProGuard by name, because it’s still the main tool for Android development, but these arguments apply just the same to R8.)