We are going to post our submission as a preprint, so it will be confusing for readers -- they may think the paper has already been submitted. I don't see how to remove this or turn the paper into a draft (rsos_draft is not recognized)
This is included in rsos.clsclass file provided by RSOS here https://royalsocietypublishing.org/rsos/for-authors
I don't see any option or configuration on ho to remove it. You need to ask them, or use another format for preprint which would use another class file. (rticles format is using the rsos.cls file )
You should contact them to understand / learn how to hide this box for pre print.