Both SlimSocial and Frost are Facebook wrappers that enable FB Messenger use without installing Facebook's official Messenger app. They need to be able to send notifications to function properly. I've used both apps on previous LineageOS and stock Android installs and both work perfectly.
On GrapheneOS, I do not get any notifications. For some reason, GrapheneOS does not allow me to toggle to allow either app to send me notifications--the setting is greyed out and cannot be toggled on. Why is this setting forced off without any control to manually turn it on? How can I override this to allow these apps to send notifications? Is there a developer option to get around this? For reference I do have sandboxed google play installed, but I doubt it's relevant to this issue either way.