Not really. It has nothing to do with LaTeX. Most of them are italic in mathematics . For example, in Hermite's textbook (1882) (https://gallica.bnf.fr/ark:/12148/bpt6k97981084/f31.item.tex...), page 13, the dω is italic. In Klein's Lectures on Mathematics (1894) (https://www.mathunion.org/fileadmin/ICM/Proceedings/ICM1893....) page 20, the dx is italic. In Goursat's textbook (1933) (https://gallica.bnf.fr/ark:/12148/bpt6k9454797/f107.item), the dx is also italic.