The NSC is essential to strategizing and formulating our foreign policy. Historically we've deliberated with the State Department on a private Discord server, with the SoS scheduling meetings for appointed members.
Back when I was originally SOS and during my time on the NSC before that, I believe our meetings were held on Discord as well, often as a channel of the Administration's server. Given the rapidly changing nature of foreign policy and the large number of people I would be contacting if confirmed, using Discord seems like the only realistic option.
Prior to discord, the NSC was conducted on Mibbit back to the Blair administration.