Is it possible to have subscribers automatically renew their membership using either the PayPal or Stripe gateway platforms by default?

I read the help guide, I am a little unclear if it will if the user saves their card details with this function of IP.Commerece/Nexus

Also, any clarification if the issue regarding unpaid/expired membership renewals has been fixed?

 

Yes, it is possible. For Stripe and credit cards, the user gets a checkbox to store the card authorization. For PayPal, there are the so-called Billing Agreements and when you set up the payment method, you choose if that is turned on, off or optional.