Where to write hook_commerce_payment_method_info() in drupal8?
Payment methods are now called Payment gateways and they are plugins, like anything else.

So, look for the @CommercePaymentGateway annotation, like in this example: https://github.com/drupalcommerce/commerce/blob/8.x-2.x/modules/payment_example/src/Plugin/Commerce/PaymentGateway/Onsite.php and the others in that folder.