What do I do with my backpack while doing True Dungeon?

Either wear it (if not bulky, I usually wear a drawstring bag) or leave it at your hotel.  I believe they still do a bag check but that is for Adventurer's Guild which constitutes your $2,500 or above token buyers.  That was the case in previous years but I haven't checked lately.  

Every time I think I have heard a surprisingly high dollar amount relating to True Dungeon expenditures, I see another higher one.

$2000...for bag check? I'm sure there are other perks too, but...spend $2000, or have your team haul your backpacks through our set pieces, or don't bring your backpack...to a day at the gaming convention? That's nuts. Just plain nuts.

You can get the badge that grants you bag check access for around $5 on the secondary market.

Man...  I gotta figure out a way for Nascrag to sell tokens.

You'll attract the kinds of players who think they should be able to "win" your game because they've spent the most money.

I recommend taking the time to leave your bag in a Hotel room or locked securely elsewhere. 

Maybe someone needs to have a locker service as an event? That might help a lot of people if they can figure out a way to manage the liability. 

With all the possibilities of illegal substances or even harmful things, I would be surprised if anyone could afford the insurance such an event would cost. 

As for locking it securely elsewhere, do you have any suggestions on where you could do that?

If there were and are no more, why would they vanish?  I'm guessing it's for the same reason that a lot of public trash cans and postal boxes have vanished and concrete bollards have proliferated.  And the convention center itself having liability is easier than any individual event organizer or even the con itself.  

I'm skeptical that liability/insurance is the barrier.  Self-serve lockers are in lots of other places -- gyms, malls, swimming pools, ski chalets, locker rooms, etc.  They post a use-at-your-own-risk sign and are done with it.

More likely, space constraints and/or maintenance would be a bigger concern.  Or just that the majority of other events held at the convention center simply don't need them in the way that Gen Con attendees might use them.