After its Emergency messaging feature launched in 2023, Apple announced during its 2024 WWDC conference the availability of simple satellite messaging with IoS18, available later in 2024. This feature is quite simple: under cellular coverage, nothing changes but outside of network coverage, just point the Iphone to the sky until a satellite is detected to be able to send a message. Et voilà! With imessage (to another iphone), the data stays crypted, which may not be the case for a regular SMS.
This service using the same satellites as the SOS service: the Globalstar constellation. Given the fact that pointing towards a satellite is needed and that most users are under cellular coverage, it is probable that usage will remain limited. It is still a good complement to the SOS feature: send an SOS and then text to your family and friends to say you are OK. This annoucement is probably also to stay ahead of the game as Samsung announced recently similar features on its future Galaxy S24.