The earliest examples of dress shields I have found in garments date from the early 1890s. However, Elizabeth Emerson on a chat group I follow (Fashion Historians United) found a reference to them being a new idea in 1863 when Godey’s Lady’s Book wrote about them:
The New Dress “Shields”
Ladies who perspire freely, and thus so soon destroy light silk, and other dresses, by discoloring them under the arms, will find complete protection by using our light and convenient ‘Shields,” made of a new material, and perfectly adapted to their use. They can be applied in an instant and taken in and out without any trouble, and add no encumbrance, which can be inconvenient or disagreeable to the most fastidious… the cost is so trifling, only twenty-five cents per pair…will save a dress worth as many dollars, it is worth while to employ it in these days of poor goods and high prices. Is it not so?
I couldn’t find any patents for these first examples, but there were plenty afterwards, starting in the 1870s: