It still is. A lot of IoT and "smart" devices are still dreadfully slow in updating their software.
Iot isn't what's holding ipv6 back. There are tons of websites and even cloud platforms with totally ipv6-capable hardware and OSes, but the services are v4-only or strongly v4-preferred.
Tbf there are always bad implementations. Good "smart" devices nowadays should be based on the Matter standard, which natively uses IPv6 addressing
https://en.wikipedia.org/wiki/Matter_(standard)
Iot isn't what's holding ipv6 back. There are tons of websites and even cloud platforms with totally ipv6-capable hardware and OSes, but the services are v4-only or strongly v4-preferred.