"When I access a website on its usual domain name “http://xyz.com”, I am automatically routed to the IPv4 version of the site. To approach the IPv6 version (if it exists), I need to type explicitly “http://ipv6.xyz.com”. This is too cumbersome."

Using ipv6 prefixes was happening often in the earlier days of IPv6. Occasionally those can still be found, but in general the URL of websites is the same regardless of the IP version used for connecting to the website. However, the few cases in which the ipv6 prefix is still present set a bad example for users, so this should be discouraged.