Lucki opened the ticket and wrote:

Right now a new folder for configurations and userdata is created in `$HOME/.openttd`.
To be more flexible it would be nice to implement the XDG Base Directory Specification 1.
This is partly related to #6603.

`$XDG_DATA_HOME` defines the base directory relative to which user specific data files should be stored. If `$XDG_DATA_HOME` is either not set or empty, a default equal to `$HOME/.local/share` should be used.
`$XDG_CONFIG_HOME` defines the base directory relative to which user specific configuration files should be stored. If `$XDG_CONFIG_HOME` is either not set or empty, a default equal to `$HOME/.config` should be used.
`$XDG_CACHE_HOME` defines the base directory relative to which user specific non-essential data files should be stored. If `$XDG_CACHE_HOME` is either not set or empty, a default equal to `$HOME/.cache` should be used.
An easy explanation can be found in the gnome wiki [2].

This would result in the following folders:

- `$XDG_CONFIG_HOME/openttd/<HOTKEYS.CFG|OPENTTD.CFG|WINDOWS.CFG>` defaulting to `$HOME/.config/openttd/<HOTKEYS.CFG|OPENTTD.CFG|WINDOWS.CFG>` when `$XDG_CONFIG_HOME` is not set or empty.
- `$XDG_DATA_HOME/openttd/<AI|BASESET|GAME|HS.DAT|NEWGRF|SCENARIO|SAVE|SCREENSHOT>` defaulting to `$HOME/.local/share/openttd/<AI|BASESET|GAME|HS.DAT|NEWGRF|SCENARIO|SAVE|SCREENSHOT>` when `$XDG_DATA_HOME` is not set or empty.
- `$XDG_CACHE_HOME/openttd/<CONTENT_DOWNLOAD>` defaulting to `$HOME/.cache/openttd/<CONTENT_DOWNLOAD>` when `$XDG_CACHE_HOME` is not set or empty. - I'm not sure about this one.

Advantages are:
- No predefined paths. The user can decide where his or her data is located.
- No cluttered home directory. Some selectors will automatically show dotfiles, finding the files you want will be easier.
- A shared storage location for data. Application data does not have to be searched for, as these are no longer located in different places. This also benefits a backup.
- Resetting the settings and keeping the application data at the same time becomes easier, as there is a clear separation.

[2]: https://wiki.gnome.org/Initiatives/GnomeGoals/XDGConfigFolders# Guidelines

Reported version: other
Operating system: Linux

This issue was imported from FlySpray: https://bugs.openttd.org/task/6630