It should, but: * I don't know any browser or app that display a special, informative message in that specific case. * You need a DNS server that answers AAAA

It should, but: * I don't know any browser or app that display a special, informative message in that specific case. * You need a DNS server that answers AAAA record to detect this. Some ISPs do not provide IPv6 connectivity, nor do their DNS servers provide AAAA records. In most places I know, especially when talking about individuals, people use their ISP-provided DNS servers.