You are quite right, a transmitter that is "off" doesn't necessarily mean that a bird is dead. However, a transmitter goes "off" because the technology fails or because the bird dies and the transmitter can't transmit anymore.
Because you are asking after them, Rain's transmitter stopped transmitting in Arkansas in October and Rosser's stopped transmitting in December in Texas after he travelled back up from Mexico. When their transmitters went offline no bodies - dead or alive - were found so they could be still alive but 7 out of 10 peregrine chicks die in the first year, so it is just as likely that the transmitters went offline because they died. At this point the only way to know if they survived is for someone to see (or have seen) them alive and to report their band numbers. And we haven't had any reports but sometimes there is a time lag so we leave their status as "unknown" in our records.