I could add a switch to quiet diplomacy events when the player isn't involved in that stuff (doesn't have own faction and isn't commissioned with anyone), and/or filter out the ones not affecting player/commissioner faction, at least.
Making these messages require comm relay access would be worse I think, they'd just contribute to (and get lost in) the usual flood of messages when re-entering comm relay range...
Anything would be nice honestly. Once one adds in a few modded factions it's basically a never ending screeching of notifications as you get pinged for every tiny detail. Feels like anytime someone sneezes in the Core Worlds I get a notification and that sound is like nails on a chalkboard.
Alternatively too, might add a toggle somewhere in the config that lets the end user just toggle themselves as well. Then if someone doesn't want those notifications, even if they have a faction, they can just quiet them. Or if someone is keen to know every detail when unassociated they can likewise toggle.