Each of your relays must support note deletion: NIP09
I've tested https://nostr-delete.vercel.app/ a few times, never had any luck.
Most recently, my "Test" note15a6ufyzezt7n5rqevulfv2quwa7yr58t674pr9gt3j24fl04s2dsc33yhy was put into the site above, enter. It seems to "go through." Yet, over at https://nostrexplorer.com the event a775c4905912fd3a0c19673e96281c777c41d0ebd7aa11950b8c9554fdf5829b shows it's still there (plus, it's still there in my notes feed).
Nothing else...just sayin'.
#nostr
Discussion
Thanks for the info (and for the recommend of Tasks.org app). Looking at NIP-09, seems like there's a few qualifiers for the note to get deleted. I'm thinking it's best to well assume notes are permanent. (Pretty much the case always on the internet!)