作为@ MH-cbon建议简单地检查，如果指针不nil为*bool足以确定该标签的存在。但是我去了，然后将XML响应结构转换为Response仅包含所需信息Status的常量结构，并将其转换为常量。

所以现在是：

// Raw XML DTO
type XmlResponse struct {
    XMLName      xml.Name  `xml:"domain"`
    Authority    string    `xml:"authority,attr"`
    RegistryType string    `xml:"registryType,attr"`
    EntityClass  string    `xml:"entityClass,attr"`
    EntityName   string    `xml:"entityName,attr"`
    DomainName   string    `xml:"domainName"`
    Status       XmlStatus `xml:"status"`
}

// Raw XML DTO
type XmlStatus struct {
    Active    *bool `xml:"active,omitempty"`
    Available *bool `xml:"available,omitempty"`
    Invalid   *bool `xml:"invalid,omitempty"`
}

// Return Response struct which doesn't have the unnecessary XML
func (x XmlResponse) GetResponse() Response {
    st := Invalid // Default to invalid

    if x.Status.Active != nil {
        st = Active
    } else if x.Status.Available != nil {
        st = Available
    } else if x.Status.Invalid != nil {
        st = Invalid
    }

    return Response{
        Domain: x.DomainName,
        Status: st,
    }
}

// Proper response parsed from XML
type Response struct {
    Domain string
    Status Status
}

type Status uint8

const (
    Invalid Status = iota
    Active
    Available
)

解析发生为：

var xmlresp XmlResponse
err := xml.Unmarshal([]byte(str), &xmlresp)
if err != nil {
    panic(err)
}

ans := xmlresp.GetResponse()

fmt.Printf(`%#v`, ans)