I've seen this in some tex file where a word is set in italics, but its ending is in regular font. Trying it out I don't see it making any difference, so I must be missing something. What does the \/ do?
a mwe:
\documentclass{article}
\begin{document}
so many \textit{test}\/s
so many \textit{test}s
\end{document}
\textitalready inserts the\/. You should compare the output with{\itshape test}sto see the difference. – Ulrike Fischer Mar 03 '15 at 11:29