Hi,
Most payment plugins actually redirect the customer to the payment gateway for entering the credit card at the end of the checkout and thus don't require that.
Only a handful of them ask the credit card data during the checkout, like the MIGS plugin and indeed, in that case, PCI DSS is strongly advised on the hosting.
Having a credit card form sending the credit card data to the payment gateway in the middle of the checkout is tricky. The issue is that the checkout pages are a form already. And having a form in a form is not possible. That's why the iframe is a good compromise, but not all payment gateways offer that capability. Some also propose to have the credit card form at the end of the checkout, in which case there is no checkout form anymore.
Basically PCI DSS compliance should be asked for payment plugins where you enter the credit card data in the middle of the checkout: PayPal Pro, Authorize.net in AIM, MIGS, the "credit card" payment plugin, eSelect, iPayDNA, iveri, PayJunction, Payment Express in PxPost, VirtualMerchant, WestPac in API, WorldNetTPS.
As you can see, for 4 of them, there is an alternative solution with the same payment gateway (another API which doesn't collect the credit card during the checkout).