They do disclose them when they are relevant to a story. For example, https://www.npr.org/2025/10/08/nx-s1-5564684/macarthur-overd... and https://www.npr.org/sections/goats-and-soda/2025/04/15/nx-s1.... They also mention them very frequently on air, anybody who has listened to NPR for more than like ten minutes probably heard them mention the Robert Wood Johnson Foundation.

You can see a full list of donors in their latest annual report: https://media.npr.org/documents/about/annualreports/2024%20A...