The default/core payment gateways in Magento 2 check if user has accepted
terms of service before allowing checkout.
There is a sample payment gateway here for Magento 2: https://github.com/magento/magento2-samples/tree/master/sample-module-payment-gateway
It doesn't implement
terms of service validation and even if they are activated, they are not displayed for this payment gateway => the user can checkout without agreeing with the TOS.
There is some doc about field validators but nothing about the TOS validation in particular. The TOS checkbox is a bit different from other fields as it doesn't seem to be created in the module template files.
How is the TOS validation supposed to be implemented in a custom payment gateway?