That door alarm thing that has a Windows XP workstation VM the facilities team touches once a month probably doesn't support IPv6.

Repeat that scenario across multiple BUs and multiple locations and no leader wants to commit to doing that kind of due diligence. What's wrong with our current IP?

Windows XP supports IPv6.

Man in the middle certificate re-signing deep packet inspection firewalls are notorious for not supporting IPv6. Most everything else has switched, but many network admins fear IPv6 and don't want to have to learn something new.

"Made today"? Probably not. "Still in operation today"? Definitely.

My company makes what is essentially an enterprise IoT device. I'd guesstimate 10% of networks with our hardware in them have no ipv6 support at all. And these are businesses that are on the more tech savvy side (I would assume, since they're ordering our stuff).

It is not about the gear, it's about security people that force you to disable IPv6. "You do not have a valid technical or business reason to use it. And, as electricians say, a VISIBLE circuit break provides the best assurance that this circuit will not kill you. Lack of IPv6, as opposed to just firewalling it, is the equivalent of the visible circuit break. I would also enable a whitelist of permitted ethertypes on all switches, and not include IPv6 there."

And let me quote from CIS SUSE Linux Enterprise 15 Benchmark v1.1.1 page 191: "3.1.1 Disable IPv6 (Automated). Profile Applicability: Level 2 - Server, Level 2 - Workstation."

Having an Allowlist of ethertype (ARP/IPv4/IPv6) is an extremely good idea IMO, as Windows and Linux are extremely permissive in what they accept on L2: https://blog.champtar.fr/VLAN0_LLC_SNAP/

TP-Link enterprise wireless access points (the EAP line) don't support IPv6.