Automatic Fake User is a Blender add-on from carlosmu that handles a common annoyance: forgetting to enable the fake-user flag on a datablock and losing it. When you save your file, the add-on assigns fake-user automatically, and in its first version it covers actions, materials, geometry nodes, shader groups and worlds.
The add-on is fully configurable through its preferences. You can exclude specific datablocks by name, suffix or prefix, which helps when you are testing features and do not want everything preserved. The exclusion marker is adjustable too, so the default "exclude" text can be swapped for a symbol such as "!" or "#". If you prefer a non-automatic workflow, you can deactivate the on-save assignation and run the assignment manually from a button in the File Menu.
It supports Blender 3.6 through 4.4, is marked as a Blender Extension, and ships under the GPL license. The developer also notes that further data types can be added in future versions on request. Documentation is included, and a demo video is available.