IPv6 is already used by a lot of enterprises in their infrastructure. It's in real world use from enough time ago that i don't think it make sense to have the support disabled by default, (at least on the server).

These days all the operating systems the zabbix server run on support IPv6 out of the box and we can also think to remove a lot of preprocessor code that check for this. Also, we use LibCURL that will, probably, have IPv6 always enabled