There is no traditional "custom firmware" (like OpenWrt or Tomato) that can be fully flashed onto an Apple Time Capsule due to Apple's tightly locked, digitally signed bootloader . However, the device runs on a NetBSD-based platform
build on top of it. This allows the device to continue working with apple time capsule custom firmware
By flashing open-source operating systems onto the Time Capsule’s aging hardware, you can transform it from a deprecated relic into a powerful, modern router, an ad-blocking DNS server, a VPN gateway, or a network-attached storage (NAS) device. This article dives deep into why, how, and what to expect when installing custom firmware on your Apple Time Capsule. There is no traditional "custom firmware" (like OpenWrt