"Theoretically it is possible. All relays would have to agree at once to delete your content." - Very unlikely scenario (e.g. I use more than 20 relays), but it just means you posted something horrendous and should be censored - I would call it moderation.  

 But somehow all of my relays lost my metadata event and my displayname have changed to the previous one 🤪. Use paid relays if you don't want your events to be deleted for reduction of storage  costs.