That looks to be a recent interpretation of an 1884 law that could be reversed by the current administration, if it wanted to make shutdowns smoother, as easily as it was adopted in the 1970s.

(And, if there were a general consensus/expectation within an agency/administration, it could be a 'dead letter' law even without explicit re-interpretation. OK, so you "break the law" by answering an email, sending an official tweet, or staffing a booth during the shutdown. Which of your fellow federal employees is going to prosecute you once the shutdown ends?)