GET https://{instance}/{collection}/_apis/wit/workitemrelationtypes?api-version=5.0

{attributes: {…}, referenceName: "System.LinkTypes.Related-Forward", name: "Related", url: null, _links: null}
{attributes: {…}, referenceName: "System.LinkTypes.Hierarchy-Forward", name: "Child", url: null, _links: null}
{attributes: {…}, referenceName: "System.LinkTypes.Hierarchy-Reverse", name: "Parent", url: null, _links: null}
{attributes: {…}, referenceName: "System.LinkTypes.Dependency-Forward", name: "Successor", url: null, _links: null}
{attributes: {…}, referenceName: "System.LinkTypes.Dependency-Reverse", name: "Predecessor", url: null, _links: null}
{attributes: {…}, referenceName: "Microsoft.VSTS.TestCase.SharedStepReferencedBy-Forward", name: "Test
 Case", url: null, _links: null}
{attributes: {…}, referenceName: "Microsoft.VSTS.TestCase.SharedStepReferencedBy-Reverse", name: "Shared
 Steps", url: null, _links: null}
{attributes: {…}, referenceName: "Microsoft.VSTS.Common.TestedBy-Forward", name: "Tested
 By", url: null, _links: null}
{attributes: {…}, referenceName: "Microsoft.VSTS.Common.TestedBy-Reverse", name: "Tests", url: null, _links: null}
{attributes: {…}, referenceName: "System.LinkTypes.Duplicate-Forward", name: "Duplicate", url: null, _links: null}
{attributes: {…}, referenceName: "System.LinkTypes.Duplicate-Reverse", name: "Duplicate
 Of", url: null, _links: null}
{attributes: {…}, referenceName: "Microsoft.VSTS.TestCase.SharedParameterReferencedBy-Forward", name: "Referenced
 By", url: null, _links: null}
{attributes: {…}, referenceName: "Microsoft.VSTS.TestCase.SharedParameterReferencedBy-Reverse", name: "References", url: null, _links: null}
{attributes: {…}, referenceName: "Hyperlink", name: null, url: null, _links: null}
{attributes: {…}, referenceName: "ArtifactLink", name: null, url: null, _links: null}

{attributes: {…}, referenceName: "Microsoft.VSTS.Common.Affects-Forward", name: "Affected by", url: null, _links: null}
{attributes: {…}, referenceName: "Microsoft.VSTS.Common.Affects-Reverse", name: "Affects", url: null, _links: null}

Explanation:

API version can be specified either in the header of the HTTP request or as a URL query parameter:

HTTP request header: Accept: application/json;api-version=1.0
Query parameter: GET {url}/{organization}/_apis/{area}/{resource}?api-version=1.

Supported versions