Why did you replace the Extremes?You're the one to ask... are there other non-Apple consumer grade routers that do support roaming assistance standards 802.11k,v or r? If not and if Apple is out of the router business(?), then this would leave iPhones out of standards-based roaming assistance.
We track 11kvr and other roaming assistance and many other attributes in the Wi-Fi System and AP Finders. kv support is most common in Wi-Fi/Mesh systems and enterprise-grade APs. Support in lower end APs is not so much.