These are provided for informational and archival use only.
No Mac NDK before NDK r21 was signed or notarized. This shouldn't matter for NDK downloads performed by Android Studio's SDK Manager, but if you manually download an NDK from this page using a web browser, on macOS 10.15 you'll see warnings when you try to run the extracted tools. You can override this in the "Security & Privacy" section of Settings. See Apple's Safely open apps on your Mac documentation, specifically the section "How to open an app that hasn’t been notarized or is from an unidentified developer" on that page.