"The aufs-dkms package is not part of bullseye. Most aufs-dkms users should be able to switch to overlayfs, which provides similar functionality with kernel support. However, it's possible to have a Debian installation on a filesystem that is not compatible with overlayfs, e.g. xfs without d_type. Users of aufs-dkms are advised to migrate away from aufs-dkms before upgrading to bullseye."
"The aufs-dkms package is not part of bullseye. Most aufs-dkms users should be able to switch to overlayfs, which provides similar functionality with kernel support. However, it's possible to have a Debian installation on a filesystem that is not compatible with overlayfs, e.g. xfs without d_type. Users of aufs-dkms are advised to migrate away from aufs-dkms before upgrading to bullseye."
https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.en.html#noteworthy-obsolete-packages