Citizen22 This seems to indicate the GOS devs have a way to be alerted to silent SMS.
I do not see how to read that text as indicating that.
Note that the FAQ text includes "Receiving a silent SMS is not a good indicator of being targeted by your cell carrier, police or government". Personally I read that text as indicating that the GrapheneOS developers are not any more concerned by "silent" SMS than non-"silent" SMS, and thus have not written any code to treat "silent" SMS messages in any special way.
High-threat targets might sensibly be concerned about SMS attacks. But high-threat targets should also be concerned about threats to any and all parts of the telephony stack. For example, last year there was a remote code execution vulnerability in the cellular baseband code on some Pixel device (source). That vulnerability did not require silent SMS messages, non-silent SMS message, silent MMS messages, etc. It was a bad bug that had nothing to do with SMS. Arguably, high-threat targets should disable the cellular modem entirely ("airplane mode"). As a side effect, that will ensure zero risk from "silent" SMS messages.
My understanding is that so far the GrapheneOS developers do not view "silent" SMS messages as a threat worth writing code about. Please note that I do not speak for the GrapheneOS project.