Renegade3 I understand this has been somewhat answered by @GrapheneOS that they do not plan to, or are unable to, implement this.
So far I don't think it's known whether or not it's possible, let alone how difficult it might be. See below.
Renegade3 T-Mobile is now partnering with Starlink to roll out satellite text capability this year with voice and data in the future. I would really like to use this as well, and hope that it doesn't have similar technical restrictions as the stock SOS messaging and is able to be utilized with GrapheneOS.
Again I don't believe it's known what would be necessary for this to work. Terrestrial cellular protocols are documented and multiple implementations exist. So far the satellite situation seems to be based on "partnerships", which appears to mean tightly controlled access. It would be one thing if Garmin/Skylo or T-Mobile/Starlink approached the GrapheneOS project about a partner relationship, but I am unaware of that happening. Perhaps users who really want this could tell those companies that!
Meanwhile, if this is technically feasible, there is a question about the engineering labor it would take. The satellite-cellular space is changing rapidly, so there is no guarantee that work done to get something working on the Pixel 9 would also work on the Pixel 10.
If I recall correctly, the built-in thermometer on some Pixel 8 and 9 devices gained GrapheneOS support after the Pixel 9 devices came out. It would make sense if the developers had held off on investing in that feature until they were aware it wasn't a one-off (the way that one of the Pixel 4's had dedicated hardware for face recognition, but not any device since).
Renegade3 While I realize there are dedicated devices for this, these features would be invaluable for those of us who do not live in or near a population center.
Personally I would recommend that nobody expect GrapheneOS to have any satellite support for a time measured in years. Maybe only one year! Or maybe only after the market settles down a lot, which will probably be multiple years. And it might very well happen that any eventual support would be for some later version of the technology, i.e., if the Pixel 11 has a satellite calling feature that works with GrapheneOS, it might still happen that the satellite features of the Pixel 9 will never be supported by GrapheneOS.
Please note that I do not speak for the GrapheneOS project.