I have never been spanked with a slipper or sandal. I can not recall using one either. Like the cane, I think it is a common instrument in Great Britain. Not so much in the USA.
I recall a childhood chum’s Mom using her wooden sandal on him. She caught us smoking at age 14. I thought she was going paddle me with it also. Based on the howls I heard from him, she knew how to use it. Darn handy to have a instrument so convenient to use.
Like slippers, canes and gym pumps are almost unknown in the states. (The generic for gym pumps was tennis shoes when I was a lad. That was before Nike, etc. and $200 shoes)
The popular instruments here are the hairbrush with anything else a distant second. Then in some order, belts, rulers, wood spoons, paddles. Paddles are the only instrument used in schools and not that many schools anymore.
All of the slippers I have seen here would not faze a flea. Wooden sandals yes.