The original spec (nip04, still used on Damus and Primal afaik) leaks metadata. Meaning the content is encrypted but others can see who sends messages to who at what time. Nip17 fixes this and is supported by a bunch of clients like Amethyst, key chat, yakihone and others. Main issue from a user point of view is that there's now multiple specs that are not compatible, so depending on the client people use they might not see DMs from people from the other client. Nip17 is superior in terms of privacy so unfortunate that not all clients support it after all that time.