gMan it seems repeating notifications for missed calls/messages is standard in AOSP or Google's version and the setting to enable it is in notification settings > advanced, however this is missing in GOS, is that correct?
I don't know whether or not this is a feature of Google's OS. It seems unlikely that the GrapheneOS team would delete such a feature if it were part of AOSP, so I assume it isn't.
gMan for at least some people, this is hardly an optional feature - repeating notifications for missed messages/calls is absolutely a required functionality - even many cheap land-line phones have this
It seems plausible that there are apps for this.
gMan is this something GOS intends to implement?
I would not expect that. The GrapheneOS project is focused on security and privacy, and does not typically invest in UI extensions or tweaks.
Please note that I do not speak for the GrapheneOS project.