Can Telegram notifications be disabled?
First, AFAIK, "Notification Service" makes reference to receive notifications in Telegram about every new update (changelog or "What's new"), as well as another announcements which Telegram staff may consider important for you to know via Telegram chat.

Second, Android Lollipop has a new way to manage notifications per app. Enter Settings screen for your device and follow these steps:

 

 

Note: Screenshots taken from my Sony Xperia Z1 running Android 5.0.2,  appearance may vary for your device.