I agree that this might not be the best solution, but it is not wrong to have admin_level on boundary=historic because it gives you more info what that historic boundary was. I would not delete it so easily just because someone could make an error. 

There are lot of errors where administrative tag is deleted just because ID editor has "not so friendly" user interface but still we can correct the issues. There is a fair amount of tools for catching and fixing these errors so I would not go on removing some tags just to prevent some things. Data should be removed just in cases when it is totaly wrong. In this case I thing that admin_level tag should be restored.