Traditional Way:

Site Settings -> Look and Feel: Navigation Elements -> Check ‘Enable Tree View’

Programmatic way using PnP Powershell:

try {
    $web = Get-PnPWeb -Includes TreeViewEnabled
    Write-Host "Setting options to enable tree view..."
    $web.TreeViewEnabled = $true        
    $web.Update()
    Invoke-PnPQuery
    Write-Host "Completed."
}
catch {
    $ErrorMessage = $_.Exception.Message
    Write-Host "Error enabling tree view. $ErrorMessage"
}

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