I know you can license a GDB which requires the end user to need license key, but is there a way to make GDBs or feature classes or shapefiles "read only" so the end user cannot edit the data? I saw somewhere that the only way to do this is to set the permissions in the Windows folder on Explorer to "Read only". Is that correct?
If managing permissions/access to data is important, the best option is to work with enterprise geodatabases. You can compress a file geodatabase, which makes it read-only, but someone could uncompress it if they want. Also, there can be issues with certain workflows/tool and compressed data. Since file geodatabases are file-system based, you can also make the folder read-only as you have already read about.
There is no security model built into file geodatabases (licensing isn't really a security model) so there is no proper way to secure them. If you shared why you want to make the file geodatabase read-only, people might be able to provide additional ideas.