"No restriction" vs. "no restrictions"

The data center must be flexible. There should be no restriction/restrictions on user's choice of protocols.

What should it be?

The way this sentence is phrased, I would select "restriction", as it refers to a single specific decision upon which a restriction would be placed:

There is no restriction on the number of times you may view your account.

If, instead, the concept of restriction was applied to multiple decisions, or to no decision in particular, then I would make it plural:

There are no restrictions associated with the account you have selected.

As an alternate example:

If the object is pluralized, then it does not apply to a specific instance:

There are no spoons on the table.

But if singular, then is refers to a specific item which is not present:

Do not try and bend the spoon. That's impossible. Instead, only try to realize the truth. There is no spoon.

In your sentence I think no restriction is enough, because it's talking about a requirement, and restriction is mentioned in general sense.

Conversely, if it's a technical doc discussing how to avoid known restrictions imposed by multiple softwares, no restrictions is better.