Sending messages using One-time Pads are perfectly secure when used properly, which is pretty simple - don't reuse the keys and the message has to be shorter than the key you are using.
And is there an app or service in existence currently using OTPs "properly", as you say?
OTPs are extremely onerous, if not impossible to implement properly in real life in a way that scales. Hence why nobody uses them (instead people use RSA/ECC/AES/etc).
https://en.wikipedia.org/wiki/One-time_pad