- The attribute `mpi` is now consistently used to provide a default, system-wide MPI implementation. The default implementation is openmpi, which has been used before by all derivations affects by this change. Note that all packages that have used `mpi ? null` in the input for optional MPI builds, have been changed to the boolean input parameter `useMpi` to enable building with MPI. Building all packages with `mpich` instead of the default `openmpi` can now be achieved like this:

  ```nix
  self: super:
  {
    mpi = super.mpich;
  }
  ```

- The Searx module has been updated with the ability to configure the service declaratively and uWSGI integration. The option `services.searx.configFile` has been renamed to [services.searx.settingsFile](options.html#opt-services.searx.settingsFile) for consistency with the new [services.searx.settings](options.html#opt-services.searx.settings). In addition, the `searx` uid and gid reservations have been removed since they were not necessary: the service is now running with a dynamically allocated uid.

- The libinput module has been updated with the ability to configure mouse and touchpad settings separately. The options in `services.xserver.libinput` have been renamed to `services.xserver.libinput.touchpad`, while there is a new `services.xserver.libinput.mouse` for mouse related configuration.

  Since touchpad options no longer apply to all devices, you may want to replicate your touchpad configuration in mouse section.

- ALSA OSS emulation (`sound.enableOSSEmulation`) is now disabled by default.

- Thinkfan as been updated to `1.2.x`, which comes with a new YAML based configuration format. For this reason, several NixOS options of the thinkfan module have been changed to non-backward compatible types. In addition, a new [services.thinkfan.settings](options.html#opt-services.thinkfan.settings) option has been added.

  Please read the [ thinkfan documentation](https://github.com/vmatare/thinkfan#readme) before updating.

- Adobe Flash Player support has been dropped from the tree. In particular, the following packages no longer support it:

  - chromium

  - firefox

  - qt48

  - qt5.qtwebkit

  Additionally, packages flashplayer and hal-flash were removed along with the `services.flashpolicyd` module.

- The `security.rngd` module has been removed. It was disabled by default in 20.09 as it was functionally redundant with krngd in the linux kernel. It is not necessary for any device that the kernel recognises as an hardware RNG, as it will automatically run the krngd task to periodically collect random data from the device and mix it into the kernel's RNG.

  The default SMTP port for GitLab has been changed to `25` from its previous default of `465`. If you depended on this default, you should now set the [services.gitlab.smtp.port](options.html#opt-services.gitlab.smtp.port) option.

- The default version of ImageMagick has been updated from 6 to 7. You can use imagemagick6, imagemagick6_light, and imagemagick6Big if you need the older version.

- [services.xserver.videoDrivers](options.html#opt-services.xserver.videoDrivers) no longer uses the deprecated `cirrus` and `vesa` device dependent X drivers by default. It also enables both `amdgpu` and `nouveau` drivers by default now.

- The `kindlegen` package is gone, because it is no longer supported or hosted by Amazon. Sadly, its replacement, Kindle Previewer, has no Linux support. However, there are other ways to generate MOBI files. See [the discussion](https://github.com/NixOS/nixpkgs/issues/96439) for more info.

- The apacheKafka packages are now built with version-matched JREs. Versions 2.6 and above, the ones that recommend it, use jdk11, while versions below remain on jdk8. The NixOS service has been adjusted to start the service using the same version as the package, adjustable with the new [services.apache-kafka.jre](options.html#opt-services.apache-kafka.jre) option. Furthermore, the default list of [services.apache-kafka.jvmOptions](options.html#opt-services.apache-kafka.jvmOptions) have been removed. You should set your own according to the [upstream documentation](https://kafka.apache.org/documentation/#java) for your Kafka version.