Healthy person's ProGuard rules for Android and Kotlin.

proguard-rules.pro

# preverify is useful only for J2ME
# R8 doesn't preverify by default
#-dontpreverify

-optimizationpasses 5
# this seems to be enough

# remove indirection
-allowaccessmodification

# move everything into the root package
-repackageclasses ""

# remove source file names
-renamesourcefileattribute ""

# keep Parcelable Creators due to crappy interface design
-keepclassmembers class * implements android.os.Parcelable {
  public static final ** CREATOR;
}

# suppress warnings
-dontwarn android.support.**

# remove runtime assertions, they are enforced in compile-time by Kotlin compiler
-assumenosideeffects class kotlin.jvm.internal.Intrinsics {
  public static void checkExpressionValueIsNotNull(java.lang.Object, java.lang.String);
  public static void checkFieldIsNotNull(java.lang.Object, java.lang.String);
  public static void checkFieldIsNotNull(java.lang.Object, java.lang.String, java.lang.String);
  public static void checkNotNull(java.lang.Object);
  public static void checkNotNull(java.lang.Object, java.lang.String);
  public static void checkNotNullExpressionValue(java.lang.Object, java.lang.String);
  public static void checkNotNullParameter(java.lang.Object, java.lang.String);
  public static void checkParameterIsNotNull(java.lang.Object, java.lang.String);
  public static void checkReturnedValueIsNotNull(java.lang.Object, java.lang.String);
  public static void checkReturnedValueIsNotNull(java.lang.Object, java.lang.String, java.lang.String);
}

# remove unused string builders
-assumenoexternalsideeffects class java.lang.StringBuilder {
    public java.lang.StringBuilder();
    public java.lang.StringBuilder(int);
    public java.lang.StringBuilder(java.lang.String);
    public java.lang.StringBuilder append(java.lang.Object);
    public java.lang.StringBuilder append(java.lang.String);
    public java.lang.StringBuilder append(java.lang.StringBuffer);
    public java.lang.StringBuilder append(char[]);
    public java.lang.StringBuilder append(char[], int, int);
    public java.lang.StringBuilder append(boolean);
    public java.lang.StringBuilder append(char);
    public java.lang.StringBuilder append(int);
    public java.lang.StringBuilder append(long);
    public java.lang.StringBuilder append(float);
    public java.lang.StringBuilder append(double);
    public java.lang.String toString();
}

-assumenoexternalreturnvalues public final class java.lang.StringBuilder {
    public java.lang.StringBuilder append(java.lang.Object);
    public java.lang.StringBuilder append(java.lang.String);
    public java.lang.StringBuilder append(java.lang.StringBuffer);
    public java.lang.StringBuilder append(char[]);
    public java.lang.StringBuilder append(char[], int, int);
    public java.lang.StringBuilder append(boolean);
    public java.lang.StringBuilder append(char);
    public java.lang.StringBuilder append(int);
    public java.lang.StringBuilder append(long);
    public java.lang.StringBuilder append(float);
    public java.lang.StringBuilder append(double);
}

-assumevalues class android.os.Build$VERSION {
    int SDK_INT return 21..2147483647;
}

maybe you write what doing each line of code?

Done.