I searched around a little and I see that this setting is deprecated. I don't know what version of Struts marked this as deprecated or what effect it currently has. My project has it set to 2, which I am sure is because that is what it was set at in a previous project.
Here is the documentation that I found:
The debugging detail level that controls how much information is logged for this servlet. Accepts values 0 (off) and from 1 (least serious) through 6 (most serious). DEPRECATED - Configure the logging detail level in your underlying logging implementation.