I think that’s the important distinction here. The existing implementation already restricts ordinary users to deleting their own topic only when it has no replies:
So the narrower feature request could simply be to make the currently hardcoded 24.hours configurable, while retaining the existing posts_count <= 1 restriction. That could default to 24 hours but allow admins to choose a longer period, or potentially unlimited deletion of unanswered topics.
I don’t think GDPR is really the reason this feature is needed. The right to erasure is not absolute, and it doesn’t mean forum software has to provide an unlimited self-service option to delete any content a user has posted.
The configurability of the existing 24-hour restriction seems like a sufficient argument for the feature on its own.