Which relays are you using? What are their storage policies, and promises?
Discussion
https://nostrcheck.me/media/public/nostrcheck.me_3523554307832746631690843988.webp
Dunno if they have any specific policies or promises.
I'm guessing it's safe to assume that dms might disappear with time if you're not careful with where your data is being stored. A private relay is worth a consideration perhaps
Yes, assume that if you are not using a paid or backup private relay and you want to keep the info, you will lose it.
Assume that if you want to lose info, you will not be able to delete it if public.