tiny-initramfs-core binary package in Ubuntu Bionic amd64

 A very minimalistic initramfs implementation for booting Linux
 systems. It is designed for systems where a full initramfs (such as
 dracut or initramfs-tools) is too large, but where at least some
 functionality of an initramfs (e.g. mounting the /usr filesystem) is
 required. Without kernel modules it is less than 16 kiB in size.
 .
 tiny-initramfs has less features and is less extensible than
 full-fledged initramfs implementations, and supports only specific use
 cases.
 .
 This package contains the init binary as well as the mktirfs script
 that may be used to create working initramfs images, while not
 replacing the initramfs implementation used on the current system.

Source package