It's clear that nitrous is illegal on highway roads. However we're still allowed to have it in the car just disconnected.

But can it be used in drag strip events or racing tracks?

It's private property. That's up to the owners. They'll have officials that will inspect your vehicle either way. If they allow it, you'll have follow particular safety guidelines and make sure the racing class type allows it. They used to mark cars with nitrous for the emergency responders incase of an accident

Otherwise, that's something you'd want to ask the specific track you plan on attending. It's not really a Highway traffic act issue.

And you might want to check with your insurance company as well. Most policies are void if you take the vehicle to any track.

It is part of the highway traffic act... right? :

Nitrous oxide fuel systems prohibited - 172.1

(1) No person shall drive or permit to be driven on a highway a motor vehicle manufactured or modified after its manufacture such that nitrous oxide may be delivered into the fuel mixture unless,

(a) the part of the fuel system that may connect to a canister, bottle, tank or pressure vessel capable of containing nitrous oxide can be clearly seen by looking at the interior or exterior of the motor vehicle;

(b) there is no canister, bottle, tank or pressure vessel connected to that part; and

(c) if the part of the fuel system that may connect to a canister, bottle, tank or pressure vessel capable of containing nitrous oxide is located inside the passenger compartment, there is no canister, bottle, tank or pressure vessel capable of containing nitrous oxide in the passenger compartment. 2007, c. 13, s. 22.

Same

(2) No person shall drive or permit to be driven on a highway a motor vehicle manufactured or modified after its manufacture such that nitrous oxide may be delivered into the fuel mixture unless,

(a) the part of the fuel system that may connect to a canister, bottle, tank or pressure vessel capable of containing nitrous oxide is completely disconnected from the part of the system that connects to the engine;

(b) the disconnection can be clearly seen by looking at the interior or exterior of the motor vehicle; and

(c) the disconnected parts cannot be reconnected from inside the passenger compartment. 2007, c. 13, s. 22.

Offence

(3) Every person who contravenes subsection (1) or (2) is guilty of an offence and on conviction is liable to a fine of not less than $500 and not more than $2,000 or to imprisonment for a term of not more than six months, or to both. 2007, c. 13, s. 22.

Again, it's private property. It's not a HTA issue whether or not a specific track allows nitrous. That's up to whomever is sanctioning the event.