I believe that was only because it was mandated by the federal government as part of procurement process. It’s the same reason there was a half baked posix subsystem in Windows NT. I don’t have citations at the moment to prove my memory.
MS is major pusher of IPv6 in corporate world. Remember that when Windows 2000 was released, major v6 traffic was 6BONE overlay.
Over the next few releases, they worked hard on v6 support, and in fact some issues people had with Vista were caused by NT6 being IPv6-first OS, across all of the MS solutions. MŚ had to go backwards a bit in 6.1, introducing things like v6-over-HTTP tunnels because they found that assuming native v6+ipsec working was too much, even with Teredo.
I began my IT career in 1999 and I saw exactly one network that still had significant amounts of IPX usage, but it was already primarily IPv4.
If any vendor had tried to sell a network product in 2001 that didn't do IPv4, they would have been laughed out of the room.
Nobody is laughing in the faces of vendors in 2020 for selling products that can't do IPv6 properly... or at all.
Nobody will be laughing in 2021. Or 2022... or...
I suspect we'll be having this conversation in 2030 as well, and the same people complaining about the side-effects of carrier grade NAT four levels deep will trot out things like "IPv6 doesn't have security because it's not behind a NAT!"
One of the last pieces of software I remember using IPX exclusively was Westwood Studios "Red Alert 2" released in 2000, along with its expansion pack released in 2001
I mainly remember as Vista dropped IPX support, rendering those games unplayable on LAN's for years until someone developed a UDP patch instead
