Keeping your system-wide configuration files intact after updating SteamOS
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SteamOS 3.6 introduced a new mechanism to decide what to to keep after an OS update, and the system now keeps a list of configuration files that are allowed to be kept in the new version. The idea is that only the modifications that are known to be important for the correct operation of the system are applied, and everything else is discarded1.
However, many users want to be able to keep additional configuration files after an OS update, either because the changes are important for them or because those files are needed for some third-party tool that they have installed. Fortunately the system provides a way to do that, and users (or developers of third-party tools) can add a configuration file to /etc/atomic-update.conf.d, listing the additional files that need to be kept.