Hi,
Regarding Stripe, a completely new plugin had to be developed to comply with the changes.
Regarding PayPal, from what I could read online, you won't have anything to do unless you're using the PayPal Pro plugin:
www.jotform.com/blog/is-paypal-psd2-compliant/
www.paypal.com/uk/webapps/mpp/psd2
As PayPal Pro is only available in a few countries, most people using PayPal with HikaShop are not concerned.
Making PayPal Pro compliant will require the development of a new plugin for Cardinal. I've had a look and it's a big mess to implement.
PayPal Advanced is not mentioned, but I think you'll be fine since the payment form is still displayed by PayPal.
And for PayPal Standard, you're in the clear.
Regarding SIPS ATOS, as far as I could read no modification is required.
Basically, as long as the payment form is on the payment gateway side, it should not require any change in the payment plugin.
Still, we're working on new versions of the PayPlug and Monetico plugins as even though the payment form is on their send they require additional information to be sent to them.
So far, we haven't been notified about other payment gateways requiring a change.