Open for Verizon to read/modify, you have no way of knowing the other half’s identity (was their number given to someone else in the meanwhile), no encoding is specified/due to non-ASCII characters taking more SPACE (in 2022) so auto-SMSs still today will just force ascii text, making it often unreadable. Like, it is literally a single digit number of bytes difference.

Also, you need like.. a carrier plan to access, wouldn’t really call that open. Any network based messenger can work over my home or even McDonald’s wifi, without any third-parties