Yesterday I was required to turn on IPv6 on my router, while setting up some IoT things using Matter over Thread. Apparently that protocol uses IPv6 and doesn't work if your router is only routing IPv4.

There is a rich history of IoT devices using IPv6 to communicate among themselves without relying on the cloud. I think Nest started this trend. One Nest device sends a specific RA to make itself the router of all other Nest devices. All other devices can configure themselves thanks to SLAAC. The benefit of v6 is that there are so many addresses out there that the Nest device can just pick an arbitrary ULA and there won’t be collisions.

Don’t know about Matter though. If it requires the user to turn on IPv6 then it’s a user experience downgrade. It should just use IPv6 internally as an implementation detail.

That's incorrect. Matter-over-Thread absolutely does NOT require IPv6 on your router. Even Matter-over-WiFi will happily work in IPv4-only networks, as long as your router does not filter the IPv6 announcements.

Some routers can work as _relays_ between the Thread network and WiFi, but this is entirely optional.