In general, the IPv6 stack is not supported on many smaller mcu. It is not that options like FreeRTOS can't support the stack, but rather the resource constraints pose a challenge.
That being said, most modern SoCs are competitively priced... and will boot Linux just fine for under $5/part. =3
Some of the new IoT protocols build on ipv6 natively.
The resource overhead is minimal for modern mcus. Dropping dhcp and arp can save a lot of resources too. Also I have mcus with more ram than my first pc.
Disabling IPv6 in ESP-IDF can save about 40KiB of flash and 2KiB of RAM. Not enough that I'd do it by default, but I've hit the limit of some of my hobbyist ESP32s to the point where I've disabled modules to cram my code in there.
Disabling IPv4 saves 25KiB of flash and less than a KiB of RAM. If you're down to the last kilobytes, disabling IPv6 makes more sense than disabling IPv4. Both options are choices of last resort, though.
That being said, most modern SoCs are competitively priced... and will boot Linux just fine for under $5/part. =3