Given the recent SolarWinds breach I wonder how these networks are impacted. Most of them look like from the early 90s.
Classified military networks are very different than civilian networks. They aren't just air-gapped. Because they are not general purpose networks they can have lots of internal barriers that would not be acceptable outside of the military. Want to use HDMI for your new screen? Nope. VGA because it doesn't require compute power within the screen. Want to use a Bluetooth headset? Nope. You are stuck with a curly wire from 1972 because that wire has passed the emissions security inspections. Such principals extend to the internal barriers too. Important national security websites can look like personal websites from the 1990s not because they are not updated but because they are very restricted in how they can load information from other sources. The fact that these networks look old doesn't mean they are behind the curve on security.
Got too many passwords to remember? Want a "password manager"... lol. Good luck with that in a world where computer A isn't even allowed to be in the same room as computer B.