Am I still using APPINSIGHTS_INSTRUMENTATIONKEY anywhere?

We should be using APPLICATIONINSIGHTS_CONNECTION_STRING as an environment variable now for sending data to app insights but some of your apps may be using APPINSIGHTS_INSTRUMENTATIONKEY and you might have missed updating them.

How can you find out which apps these are?

If your using Azure Web Apps, Function Apps, Logic App Standard then the below powershell script uses Azure CLI and will help you identify apps you might have missed.

$tenantId = "<Add Tenant Here>"
#az login --tenant $tenantId | Out-Null

$subscriptions = az account list --query "[].{id:id, name:name}" -o json | ConvertFrom-Json

foreach ($sub in $subscriptions) {
    $subId = $sub.id
    $subName = $sub.name

    az account set --subscription $subId

    $webApps = az resource list --subscription $subId --query "[?type=='Microsoft.Web/sites' || type=='Microsoft.Logic/workflows']" | ConvertFrom-Json
    
    foreach ($app in $webApps) {
        $resourceId = $app.id
        $appName = $app.name
        $appKind = $app.kind
        $rg = $app.resourceGroup

        $appSettings = az webapp config appsettings list --name $appName --resource-group $rg | ConvertFrom-Json
        $setting = $appSettings | Where-Object { $_.name -eq "APPINSIGHTS_INSTRUMENTATIONKEY" }

        if ($setting) {
            Write-Host "    [-] $appName = Key Found: APPINSIGHTS_INSTRUMENTATIONKEY" -ForegroundColor Red
            Write-Host "            Web App: $appName" -ForegroundColor Gray
            Write-Host "            App Type: $appKind" -ForegroundColor Gray
            Write-Host "            Resource Group: $rg" -ForegroundColor Gray
            Write-Host "            Subscription: $subName" -ForegroundColor Gray
            Write-Host "            Subscription Id: $subId" -ForegroundColor Gray
            
        } 
        else {
            Write-Host "    [+] $appName = Key Not Found" -ForegroundColor Green
        }
    }    
}
Write-Host "Done!" -ForegroundColor Gray