- Kubernetes has [deprecated docker](https://kubernetes.io/blog/2020/12/02/dont-panic-kubernetes-and-docker/) as container runtime. As a consequence, the Kubernetes module now has support for configuration of custom remote container runtimes and enables containerd by default. Note that containerd is more strict regarding container image OCI-compliance. As an example, images with CMD or ENTRYPOINT defined as strings (not lists) will fail on containerd, while working fine on docker. Please test your setup and container images with containerd prior to upgrading.

- The GitLab module now has support for automatic backups. A schedule can be set with the [services.gitlab.backup.startAt](options.html#opt-services.gitlab.backup.startAt) option.

- Prior to this release, systemd would also read system units from an undocumented `/etc/systemd-mutable/system` path. This path has been dropped from the defaults. That path (or others) can be re-enabled by adding it to the [boot.extraSystemdUnitPaths](options.html#opt-boot.extraSystemdUnitPaths) list.

- PostgreSQL 9.5 is scheduled EOL during the 21.05 life cycle and has been removed.

- [Xfce4](https://www.xfce.org/) relies on GIO/GVfs for userspace virtual filesystem access in applications like [thunar](https://docs.xfce.org/xfce/thunar/) and [gigolo](https://docs.xfce.org/apps/gigolo/). For that to work, the gvfs nixos service is enabled by default, and it can be configured with the specific package that provides GVfs. Until now Xfce4 was setting it to use a lighter version of GVfs (without support for samba). To avoid conflicts with other desktop environments this setting has been dropped. Users that still want it should add the following to their system configuration:

  ```nix
  {
    services.gvfs.package = pkgs.gvfs.override { samba = null; };
  }
  ```

- The newly enabled `systemd-pstore.service` now automatically evacuates crashdumps and panic logs from the persistent storage to `/var/lib/systemd/pstore`. This prevents NVRAM from filling up, which ensures the latest diagnostic data is always stored and alleviates problems with writing new boot configurations.

- Nixpkgs now contains [automatically packaged GNOME Shell extensions](https://github.com/NixOS/nixpkgs/pull/118232) from the [GNOME Extensions](https://extensions.gnome.org/) portal. You can find them, filed by their UUID, under `gnome38Extensions` attribute for GNOME 3.38 and under `gnome40Extensions` for GNOME 40. Finally, the `gnomeExtensions` attribute contains extensions for the latest GNOME Shell version in Nixpkgs, listed under a more human-friendly name. The unqualified attribute scope also contains manually packaged extensions. Note that the automatically packaged extensions are provided for convenience and are not checked or guaranteed to work.

- Erlang/OTP versions older than R21 got dropped. We also dropped the cuter package, as it was purely an example of how to build a package. We also dropped `lfe_1_2` as it could not build with R21+. Moving forward, we expect to only support 3 yearly releases of OTP.