sys-kernel/aufs-sources was removed from gentoo.git as of this comment in 2020-12-29.
Aufs (Another Union File System) is an advanced multi-layered unification filesystem. Aufs was originally a re-design and re-implementation of the popular UnionFS, however after adding many new original ideas it became entirely separate from UnionFS. Aufs is considered a UnionFS alternative since it supports many of the same features.
- The ability to unite several directories into a single virtual filesystem. Calling the member directory as a branch;
- Specification of the permission flags on each branch (readonly, readwrite, and whiteout-able);
- Via upper writable branch, internal copyup and whiteout is possible (files and directories on the readonly branch are logically modifiable);
- Dynamic branch manipulation (add, delete, etc.)
Emerge the pre-patched (sys-kernel/aufs-sources) package. This will install another set of kernel sources that have had the Aufs4 patches applied. The new sources will show up in /usr/src using a aufs suffixed name scheme.
If aufs sources are not yet emerged on the system, do so presently:
emerge --ask sys-kernel/aufs-sources
After the emerge process is finished, list the available sources:
eselect kernel list
 linux-4.14.91-gentoo  linux-4.14.91-aufs
Use eselect to set the symlink to the aufs kernel sources:
eselect kernel set 2
File systems ---> Miscellaneous filesystems ---> [*] Aufs (Advanced multi layered unification filesystem) support Maximum number of branches (127) ---> [*] Detect direct branch access (bypassing aufs) method (fsnotify) ---> [ ] NFS-exportable aufs [*] support for XATTR/EA (including Security Labels) [*] File-based Hierarchical Storage Management [ ] Readdir in userspace [ ] Workaround for rename(2)-ing a directory [ ] Show whiteouts [*] Ramfs (initramfs/rootfs) as an aufs branch [ ] Debug aufs
After the features have been set, build the kernel following the kernel configuration guide.
In order to manage aufs, the sys-fs/aufs-util package is needed.
emerge --ask sys-fs/aufs-util
Check man aufs