Spectrum absolutely does support IPV6 throughout their networks. I have Spectrum and always pass end-to-end IPV6 tests to multiple sites. I do not use any Spectrum-provided equipment. I have my own modem (Arris SB6190) and router (Netgear RAXE500). The router is optimized for multiple high speed streams.
MSFS does now maintain at least some IPV6 connections if they are available. I have seen as many as 6 discrete IPV4 and up to 10 discrete IPV6 connections associated with the flightsimulator.exe process. If IPV6 is disabled (and the simplest way to do that is to simply uncheck IPV6 in your network card’s “properties” tab), then MSFS will use only IPV4.
I have never had any problems running MSFS when it is restricted to only using IPV4. Until the last few weeks, all MSFS connections were via IPV4 even when IPV6 was fully enabled on my system.
MSFS does not connect to “a” server. When the sim is running, it maintains at least 5, and sometimes as many as 10, individual TCP/IP connections to multiple servers in multiple geographic locations. Usually three of these resolve to Microsoft Azure services. For me, these are usually located in the Seattle area, though I often see a connection to a Microsoft IP addresses in Chicago.
There is always a connection to an Amazon AWS server in Portland, OR, and several to Akamai Technologies in New York City.