I like Signal and use it. I think the group behind it is pretty cool.
For those who are responding with good information and technical deets about how some platforms do or do not meet the specs I mentioned... thanks for your labor.
For those who are here to lecture about why my question is wrong... meh. I said what I said.
Platform capture of centralized services by oligarchs should be top of mind for anyone watching current events even a little bit and we should all be asking questions.