Linux Access Control Lists, or ACLs, can take some getting used to, but they're invaluable for getting a finer-grained control of your Linux filesystem permissions.

I was looking for [dm-]verity, but this guide will be useful for other subjects.

Access a virtual console, e.g. from an ssh session.

SystemRescue (formerly known as SystemRescueCd) is a Linux system rescue toolkit available as a bootable medium for administrating or repairing your system and data after a crash. It aims to provide an easy way to carry out admin tasks on your computer, such as creating and editing the hard disk partitions. It comes with a lot of Linux system utilities such as GParted, fsarchiver, filesystem tools and basic tools (editors, midnight commander, network tools). It can be used for both Linux and windows computers, and on desktops as well as servers. This rescue system requires no installation as it can be booted from a CD/DVD drive or USB stick, but it can be installed on the hard disk if you wish. The kernel supports all important file systems (ext4, xfs, btrfs, vfat, ntfs), as well as network filesystems such as Samba and NFS.

gVisor is an application kernel, written in Go, that implements a substantial portion of the Linux system call interface. It provides an additional layer of isolation between running applications and the host operating system.
Found on https://marc.info/?l=9fans&m=165401374021100&w=2

Tilck is an educational monolithic x86 kernel designed to be Linux-compatible at binary level. Project's small-scale and simple design makes it the perfect playground for playing in kernel mode while retaining the ability to compare how the very same usermode bits run on the Linux kernel as well. That's a unique feature in the realm of educational kernels. Because of that, building a program for Tilck requires just a i686-musl toolchain from bootlin.com.

Found on Leah Neukirchen's blog https://leahneukirchen.org/trivium/2021-08-15

Kiss Linux™ is a meta-distribution for the x86_64 architecture with a focus on
simplicity, sustainability and user freedom. The project was created by
Dylan Araps who also acts as its sole developer and BDFL [0].

The distribution is designed to be maintainable by a single person and to this
end has no infrastructure, backend or monetary running costs. Further, all
installations of KISS contain everything needed to continue its development.

Found via https://github.com/a-schaefers/systemE, an replacement for systemd

Found on https://ssd.eff.org/en/module/how-delete-your-data-securely-linux