A few months ago I deleted a folder with inside a release definition.
My friend Alessandro Alpi helped me a lot pointing me to this msdn article.
With the PowerShell script that you find following the link you can restore a deleted release pipeline but only within 4 weeks of the deletion.

I write down the PowerShell script here for future memory in case the MSDN blog post is moved.

param (  
 [Parameter(Mandatory=$true)]  
 [ValidateNotNullOrEmpty()]  
 [string] $token  
 )  
 ## Construct a basic auth head using PAT  
 function BasicAuthHeader()  
 {  
 param([string]$authtoken)  
 $ba = (":{0}" -f $authtoken)  
 $ba = [System.Text.Encoding]::UTF8.GetBytes($ba)  
 $ba = [System.Convert]::ToBase64String($ba)  
 $h = @{Authorization=("Basic{0}" -f $ba);ContentType="application/json"}  
 return $h  
 }  
 # Fill in your account name/project name/definition name  
 $accountName = "aseemb"  
 $projectName = "dockerConf"  
 $definitionNameToRecover = "New Release Definition"  
 # Find the Id of release definition that got deleted  
 $deletedReleaseDefinitionsUri = "https://$accountName.vsrm.visualstudio.com/$projectName/_apis/Release/definitions?api-version=4.0-preview.3&isDeleted=true&searchText=$definitionNameToRecover"  
 $h = BasicAuthHeader $token  
 $deletedDefinitions = Invoke-RestMethod -Uri $deletedReleaseDefinitionsUri -Headers $h -ContentType “application/json" -Method Get  
 $deletedDefinitionJSON = $deletedDefinitions | ConvertTo-Json -Depth 100  
 write-host "Found the deleted definitions : $deletedDefinitions"  
 $deletedReleaseDefinitionId = $deletedDefinitions.Value[0].id  
 write-host "Found the deleted id : $deletedReleaseDefinitionId " # Recover the deleted release definition  
 $undeleteReason = '{ "Comment" : "Deleted by mistake" }'  
 $undeleteReleaseDefinitionUri = "https://$accountName.vsrm.visualstudio.com/$projectName/_apis/Release/definitions/$deletedReleaseDefinitionId`?api-version=4.0-preview.3"  
 $undeletedDefinition = Invoke-RestMethod -Uri $undeleteReleaseDefinitionUri -Headers $h -ContentType “application/json" -Method Patch -Body $undeleteReason  
 $name = $undeletedDefinition.name  
 write-host "$name recovered successfully"