Official comment
ONTRAPORT handles subscriptions and payment plans internally, rather than passing the subscription information to PayPal. We have looked at allowing the use of PayPal subscriptions but the problem has always been that we cannot control the communication and status of the subscription using automation within ONTRAPORT.
The advantage of our system is that you don't have to do anything special to move from one gateway to another, as a simple support ticket will change all existing subscriptions from Stripe to Authorize.net (as an example). You don't have to contact your clients to have them re-enable their subscriptions, we just change the gateway on the backend. This allows you to change when you get better pricing on your transactions.
You can use PayPal "Buy Now" buttons for a PayPal subscription and place them in a Custom HTML block on an ONTRAPORT Page.
Edit: Forgot to mention we'll keep this open for more votes and comments to see if it's something we should investigate again.