They started doing this long before the pandemic. It's not about that.
"There is no federal statute mandating that a private business, a person, or an organization must accept currency or coins as payment for goods or services. Private businesses are free to develop their own policies on whether to accept cash unless there is a state law that says otherwise"[1]
The long standing way food trucks work is you order, wait, receive food, then pay. Now, to get around this law, food trucks have switched to pay, order, wait, receive food.
I don't think your central premise holds, however. You may be reading too much between the lines or giving too much credence to a colloquial (and perhaps incorrect) interpretation.
"when a debt has been incurred by one party to another, and the parties have agreed that cash is to be the medium of exchange, then legal tender must be accepted if it is proffered in satisfaction of that debt. However, otherwise the selling party may set the medium of exchange to be anything they choose: dollars, bananas, precious gems, feathers, whiskey, etc. They may also choose to accept cash payment only via alternative forms (e.g., credit/debit card, check, money order) rather than currency itself."
There are very real risks a food truck faces in accepting cash: fomite diseases, counterfits, armed robbery. I'm guessing you're fine enforcing the truck owners bear these risks.
For balance we all know that there are [other] risks associated in accepting card payments.
We should also note that the card payment system is not "free", not available to everyone, and does not operate without collateral damage, not least that it excludes vulnerable groups in society.
> I'm guessing you're fine enforcing the truck owners bear these risks
Enforcing following the rules (particular not wriggling around them using legal loopholes) should - one might think - be a fairly mainstream view?
I grew up in an area where Canadian currency often made it into the money supply and most vendors would just accept Canadian quarters at the US rate without a fuss. I remember the weird looks I got the first time I tried to do the same in a state further south. I feel like both are reasonable approaches but I would hesitate to legally mandate one approach because they are context dependent.
So for the loophole experience, I would need you to show if thats a viable loophole in the areas where cash payment is mandated