I'm placing tiles vertically in the bathroom (wall), non-wet areas. The wall board previously had wallpaper and in removing, gypboard exterior paper finish has been compromised, leaving brown paper in some, but not, all spots. Should I treat this in some way prior to applying thinset over it? Some kind of "primer"? Or will the thinset suit that purpose? I will clean up best as I can, but the paper appears to be absorbent, no longer waterproof.
The paper of your sheetrock never was waterproof. Some sheetrock has mold resistant paper, but certainly not waterproof. The good news is that you're not dealing with a wet area.
If there is paper on the rock still (even brown paper) your thinset should work just fine. If plain gypsum is exposed in small spots it would be advisable to at least use some thinset and mesh tape in those locations as a scratch coat prior to thinsetting your tiles in place. I don't see any reason to prime the areas in any way. The thinset will bond just fine to the paper.