これは?

以下の記述が

packagingOptions {
  resources {
    excludes += "META-INF/rxjava.properties"
  }
}

これになりました。

fun Packaging.() {
  resources {
    excludes += "META-INF/rxjava.properties"
  }
}

なんだかおかしいです?

該当部分のコードを確認します。

    /**
     * Specifies options and rules that determine which files the Android plugin packages into your
     * APK.
     *
     * For more information about the properties you can configure in this block, see [Packaging].
     */
    @Deprecated("Renamed to packaging", replaceWith = ReplaceWith("packaging"))
    val packagingOptions: Packaging

    /**
     * Specifies options and rules that determine which files the Android plugin packages into your
     * APK.
     *
     * For more information about the properties you can configure in this block, see [Packaging].
     */
    @Deprecated("Renamed to packaging", replaceWith = ReplaceWith("packaging"))
    fun packagingOptions(action: Packaging.() -> Unit)

    /**
     * Specifies options and rules that determine which files the Android plugin packages into your
     * APK.
     *
     * For more information about the properties you can configure in this block, see [Packaging].
     */
    val packaging: Packaging

    /**
     * Specifies options and rules that determine which files the Android plugin packages into your
     * APK.
     *
     * For more information about the properties you can configure in this block, see [Packaging].
     */
    fun packaging(action: Packaging.() -> Unit)

👉 build-system/gradle-api/src/main/java/com/android/build/api/dsl/CommonExtension.kt - platform/tools/base - Git at Google 

こうですね。

packaging {
  resources {
    excludes += "META-INF/rxjava.properties"
  }
}

ついでに、短くしておきます。

packaging {
  resources.excludes += "META-INF/rxjava.properties"
}

これでOK。

👉 How to replace deprecated packagingOptions in Android Gradle build files - Stack Overflow 

関連ワード:  Android・AndroidStudio・Kotlin・おすすめ・ツール・今さら聞けない・初心者・開発