It’s billing 5, but this version works too.
I uploaded 3 games (1 few weeks ago) and the Play Console didn’t say me that I needed to upgrade to V6.
I understand your frustration, I spent dozens of hours trying different versions of Unreal, of JAVA, of the SDK, of Gradle… it’s a nightmare to make mobile games on UE, because Epic just doesn’t care about mobile.
Edit: Google Play Billing Library version deprecation | Google Play's billing system | Android Developers
You have til november 2024 to use Billing V5.