@MaraEmily it's not what it sounds like.
When mandatory military service was paused years ago, the law was not stricken from the books, just "put on hold".
Now that military service is back (*), the old paragraph came back with it.
Also, it doesnt forbid anything, but if you PLAN to leave the country for a long time before you get your examination for military service, you need to tell them.

(*) also to note, military service is not mandatory in germany. You can just not do it.