Bit of an edge case here—but if all of the functions in a Retrofit interface are unused, R8 will strip them. This causes the following Proguard rule to miss:

Then once you try to instantiate the stripped service class, you get a fairly nondescript ClassCastException from Retrofit. E.g.:

Caused by: java.lang.ClassCastException
  at androidx.appcompat.app.ToolbarActionBar$$ExternalSyntheticThrowCCEIfNotNull0.m(:0)

Replacing the referenced rule with the following one fixes the issue for us:

-if interface *
-keepclasseswithmembers,allowobfuscation interface <1> {
  @retrofit2.http.* <methods>;
}

(Source: here)

Note: I barely know how to proguard, so this definitely requires some scrutiny. Raising issue here for visibility.