- Specifying [mailboxes](options.html#opt-services.dovecot2.mailboxes) in the dovecot2 module as a list is deprecated and will break eval in 21.05. Instead, an attribute-set should be specified where the `name` should be the key of the attribute.

  This means that a configuration like this

  ```nix
  {
    services.dovecot2.mailboxes = [
      { name = "Junk";
        auto = "create";
      }
    ];
  }
  ```

  should now look like this:

  ```nix
  {
    services.dovecot2.mailboxes = {
      Junk.auto = "create";
    };
  }
  ```

- netbeans was upgraded to 12.0 and now defaults to OpenJDK 11. This might cause problems if your projects depend on packages that were removed in Java 11.

- nextcloud has been updated to [v19](https://nextcloud.com/blog/nextcloud-hub-brings-productivity-to-home-office/).

  If you have an existing installation, please make sure that you're on nextcloud18 before upgrading to nextcloud19 since Nextcloud doesn't support upgrades across multiple major versions.

- The `nixos-run-vms` script now deletes the previous run machines states on test startup. You can use the `--keep-vm-state` flag to match the previous behaviour and keep the same VM state between different test runs.

- The [nix.buildMachines](options.html#opt-nix.buildMachines) option is now type-checked. There are no functional changes, however this may require updating some configurations to use correct types for all attributes.

- The `fontconfig` module stopped generating config and cache files for fontconfig 2.10.x, the `/etc/fonts/fonts.conf` now belongs to the latest fontconfig, just like on other Linux distributions, and we will [no longer](https://github.com/NixOS/nixpkgs/pull/95358) be versioning the config directories.

  Fontconfig 2.10.x was removed from Nixpkgs since it hasn't been used in any Nixpkgs package for years now.

- Nginx module `nginxModules.fastcgi-cache-purge` renamed to official name `nginxModules.cache-purge`. Nginx module `nginxModules.ngx_aws_auth` renamed to official name `nginxModules.aws-auth`.

- The option `defaultPackages` was added. It installs the packages perl, rsync and strace for now. They were added unconditionally to `systemPackages` before, but are not strictly necessary for a minimal NixOS install. You can set it to an empty list to have a more minimal system. Be aware that some functionality might still have an impure dependency on those packages, so things might break.

- The `undervolt` option no longer needs to apply its settings every 30s. If they still become undone, open an issue and restore the previous behaviour using `undervolt.useTimer`.

- Agda has been heavily reworked.

  - `agda.mkDerivation` has been heavily changed and is now located at agdaPackages.mkDerivation.