In Heinrich Mann's novel Man of Straw (in German Der Untertan), the eponymous anti-hero Diederich Hessling manages to get out of his compulsory year of military service as having flat feet. In the first letter of Letters of Heinrich and Thomas Mann, 1900-1949, Thomas Mann describes to his brother the progress of his scheme to get out of the army on the same plea. A dozen years later he summarized this for Heinrich Mann, then writing Man of Straw. Hessling got his disqualification from a medical officer who had belonged to the same student society, the Young Teutons. Thomas Mann got his through a medical officer who was friends of the family physician.
It seems unlikely that the German army lost greatly by Thomas Mann's early release from duty in 1900. No doubt he would have been on the old side to have been recalled for duty in 1914.
It is not clear to me whether Penguin keeps Man of Straw in print, but the book does not seem hard to come by. As a satire on Wilhelmine Germany it is powerful. As a picture of that society, one wonders whether it isn't exaggerated. The University of California Press has put Letters etc. out of print. I found it used, and haven't read far in it.