When it says "Instagram User" in DMs instead of showing the person's username, it usually means that the account has been deactivated, deleted, or banned. You won’t see their profile picture or be able to view their profile, but your past messages with them will still be visible. If they reactivate their account later, their username will reappear in the chat.
"Instagram User" in DMs means:
The account messaged you but either (1) deleted their profile, (2) blocked you, or (3) was banned by Instagram. You can’t view their username or reply.
When you see "Instagram User" in DMs, it usually means the person messaged you but either deleted their account, blocked you, or you unmatched (in the case of vanish mode or a deleted chat). Their username is no longer visible.
