I agree and would first urge people to test IPv6 client access using some lazy-loaded image that is IPv6 only on their website. It should look like a normal resource so that false-negatives are not introduced by NoScript or uBlock.
People can of course manually test here [1] from their homes, schools, work place. I would wager most cellular data networks would be fine. The impact would probably depend on how a website is utilized. Stats on mobile vs. non-mobile would probably be telling. The people on their cell should test LTE vs Wifi and clear cache between tests.
You would lose that wager. I’m in the southeast United States. My ISP, Spectrum, works fine with IPv6. My cellular provider, Total by Verizon, is IPv4 only.
For what it's worth I did qualify my statement using most. A few of the older cellular networks are slower to adopt and sadly I can understand why having worked for one of them. If you find it interesting here [1] are some rudimentary stats from the US. It's not an all inclusive picture of things.
People can of course manually test here [1] from their homes, schools, work place. I would wager most cellular data networks would be fine. The impact would probably depend on how a website is utilized. Stats on mobile vs. non-mobile would probably be telling. The people on their cell should test LTE vs Wifi and clear cache between tests.
[1] - https://test-ipv6.com/