iris.to disagrees!!!
Maybe it depends on the relay?
I don't know their process for deleting a note, but they definitely have an option that works. 
Discussion
You ask relays to delete .. it's up to them.
Same happens with Facebook Google and any big tech
"you delete -> no normal user see it.
BUT you don't know if that data is or is not anymore on their servers ..
since this is a decentralized protocol.. any relay can decide to "delete" posts or not.
If not it will still be visible from the corect relays.