Reading about Power Delete Suite https://github.com/j0be/PowerDeleteSuite
They don't mention being able to delete from private subs, and their method of deletion sounds like it would fail when the subs are set to private.
I'll admit Reddit barely deserves the benefit of the doubt at this point, but I have no idea how you would delete posts on private subs except through some GDPR way that must exist.
Here’s an extension to delete all of your Reddit history: https://github.com/j0be/PowerDeleteSuite