It’s a new payment plan, so applies to new student debt as well.

If I’m reading the StudentAid.gov correctly, the SAVE plan has the following:

• Increase in income exemption to 225% of poverty line (~$66k/yr)
• Undergraduate Loans now are 5% of AGI above exemption limit
• No interest accrued if you make your monthly payment
• After 10 yrs of payment, if principle loan was $12k or less, loan is forgiven. Payment period increases by 1 yr for every $1k over that amount (e.g. for $20k loan, forgiveness after 18 yrs)