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:
If, instead, the concept of restriction was applied to multiple decisions, or to no decision in particular, then I would make it plural:
As an alternate example:
If the object is pluralized, then it does not apply to a specific instance:
But if singular, then is refers to a specific item which is not present:
'no' is followed by a plural countable noun, or an uncountable noun:
There are no books on the table.
I have no money.