Cramfs

Cramfs is Article description::a memory and space sensitive filesystem that supports random reading. Ideal for use as ROM, it avoids the block device layer entirely and useful in embedded systems with very tight memory constraints. Cramfs is extremely limited in terms of features and performance.

The precursor to SquashFS, Cramfs was obsolete since late 2013 but has recently found new life from a kernel maintainer in Linux kernel 4.15.0.

Kernel
Specifically the CONFIG_CRAMFS and CONFIG_CRAMFS_BLOCKDEV options.