What do you mean by "guaranteed-issue plan"? All insurers must offer at least the same standardized minimum plans (bronze etc.). Anyone who expects a subsidy must go through an exchange.

Insurers don't HAVE to sell direct to consumer (off exchange), but they're incentivized to do so because that population is on average healthier than that coming in through the exchange. Plus it's easier to manage customer records directly than rely on the data that comes from the state or federal exchanges.