

but usually when using social media we can operate under the assumption that the delete button works.
Not at public sites. There’s several websites that mirror whole reddit where you’ll find plenty of deleted posts and comments. There’s Twitter archives as well that keep copies of all kinds of accounts and posts.
You may have better chances in semi-public social media like Facebook where you have to be logged in to see anything. But you really can’t operate under that assumption as long as you’re on public social media.
Obligated? It depends on the users a bit (since you say migrating, I guess I already know them?).
But generally a reminder won’t hurt. Not everyone gets educated on digital spaces and also we tend to forget which consequences our posts might have.
You mentioned young people as an example who should be able to fuck up without it being held against them too long and I agree. I think public social media might not be the best place for that in general, and certainly not with names that can easily be mapped to their offline id.