As this plugin is migrated to the latest billing library version in recent commits it must add the in-app messaging feature introduced in version 4.1

Here is the link to the release notes https://developer.android.com/google/play/billing/release-notes#4-1

It describes a new feature "Added BillingClient.showInAppMessages()".

In the migration to version 5.0 of in_app_purchase plugin it is missing the implementation of this feature.