How to delete a folder and all it's contents when you don't know the filenames ahead of time?

I would suggest using a Python caller with the shutil module to remove the folder directory and it's contents if you are familiar with Python.

https://docs.python.org/3/library/shutil.html#shutil.rmtree

import fme
import fmeobjects
import shutil
 
def deleteFolder(feature):
    shutil.rmtree(r"<path to folder>")

FME and Python

https://community.safe.com/s/article/python-and-fme-basics

import fme
import fmeobjects
import shutil
 
def deleteFolder(feature):
    shutil.rmtree(r"<path to folder>")

Oh. Has me wondering if I should just forego FME and do the whole thing in python (for this particular task).

Why import fme and fmeobjects if only shutil is being used?

Importing fme and fmeobjects lets you use FME's classes and methods. It isn't necessary to import fme and fmeobjects in this example though as it will still remove the directory and it's contents without them.