In particular systemd-sysext has become quite popular on immutable OSes (by which I mean the ones that provide cryptographic immutability via dm-verity, not things like ostree where the immutability is mere convention) to provide a certain level of modularity to an otherwise rigid file hierarchy.
With these tools, the /usr/ and /etc/ hierarchies become stacks (by means of overlayfs) of read-only, cryptographically protected, individually signed file system layers.