The priceValidUntil is not a new attribute and has existed for many years. However, you might think it is new due to Google’s structured data testing tool, giving you warnings since the end of last year.
When seeing warnings (not errors) in Google’s structured data testing tool it is there to notify you of optional data. This means there is no requirement, it also means there will be no SEO penalty. It also means only fill in the attribute value if it is appropriate.
The most common fix is to an end date. You can, for example, add a date that expires in a year. Make sure you do this dynamically, so that the date gets updated every year. Do not add values manually, you will forget about it and then at one point, Google will think the price expired.
The date after which the price is no longer available.
You can use the priceValidUntil attribute if you have a price drop and you know when this price drop ends.