Can a bill that was never received be collectible after 21 months?

Absolutely.

Your not really suggesting you think by claiming (or even actually) not receiving mail (for some period) means you wouldn't have responsibility to pay bills, or late fees, etc. are you?

While a legal demand for payment generally needs to be shown as being made during legal proceedings to collect the debt,(and as your now aware of the bill you've clearly received that), a bill doesn't really need to be sent or received for you to be responsible for what caused it. Most contracts and agreements even specify that any billing is just a courtesy.

You are agreeing to pay when you order/accept the service/materials or whatever. It is your contract obligation. No bill is required.