Its based on sales contract date not deposit.
Not sure about all cases dating back to '21 or early '22, but when some of us put down our deposit, in some cases it was written up as a generic contract with a fictitious vehicle entered onto the contract just to confirm that a deposit was made and a date stamp was captured to track the place on their list. For example in my case, dealer did not enter my current vehicle details (3LT, Z51, HTC, Front Lift, MRC, etc), it was just for a base car with no options. The contract details were updated once the allocation was secured, but the date remained the same (at time of deposit).
Either way, this subject is probably only valid for another year or so until everyone who put deposits prior to Sept '22 gets their car delivered. My guess is each dealer will handle things (deposits/contracts) differently. For example, for another one of my previous deposits, it was simply a screenshot of a credit card transaction that went thru, in that case there was no contract (in '21) so I would have likely paid the luxury tax had I gone thru with that dealer.
The luxury tax is a tough pill to swallow. The C8 was coined as a somewhat affordable car (compared to its exotic counterparts), with a (then) starting price of $70K CAD. Got everyone excited, and many ran out to put deposits based on that premise. Several increases later (and a luxury tax for the more expensive versions), and that same base $70K car is likely $90K today (if not more). Move to a 2LT or 3LT or HTC, or combination of any of those, plus some options, and it climbs into luxury tax territory quickly, and in some cases, may not make the C8 the affordable car it started out to be (for some).