Can I buy a pass months in advance

11 months is for a digital pass. What in case of a paper pass?

And are there also restrections for promotional passes?

With paper passes, you have to choose your start date at the time of purchase. It can still be in the future, but you must choose it.

That’s certainly true when purchased from Interrail. Possibly if you can find a vendor that sells paper passes at a ticket office, they might still sell ones where you can choose the start date later, I don’t know.

With promotional passes, sometimes they come with a shorter activation window. The current 25% sale has the normal 11 month window, but you should always check for future promotions.