I think the best translation (in terms of expressions that are actually used in German speaking countries) was mentioned by commenter "User Unknown" in the comments to the original question. He proposed
nicht jugendfrei
This is indeed the traditional and well-established expression to mark sexually explicit material.
Note that the term became widespread in times long before the emergence of the internet, and therefore warning people of materials with sexually explicit or otherwise problematic content at the workplace, of all locations, was simply not an issue. There was just no danger that somebody could involuntarily or incautiously access publications in a way that would compromise him or her vis à vis colleagues or superiors, and therefore there was no need for a special term to warn people. The only danger society anticipated was that, God forbid!, minors could see a naked breast or worse.
As for modern, internet-prone times, I have never met a German expression exactly corresponding to the "Not safe for work" euphemism. I do not know if something like this is - except for the English original - in use on German pornographic sites. Perhaps there are other people out there who can rely on a richer treasury of experience?
However, there is one related item that indeed has a German name:
Chef-Button
is used for a button on a website or in a browser to quickly close the page and display something work-related, typically a spreadsheet, so that passing hierarchs might not be offended.
(I admit that "Chef-Button" is not the best example for a German term, as the only thing differentiating it from the English original is the capitalization of initials. More Germanish would be
*Vorgesetzten-Knopf
but this I have, although a nice term, never heard in practice.)
Commenter Takkat contributed then
Cheftaste / Chef-Taste
which indeed has a very "Germanish" sound to it. I would, however, suppose, that it rather refers to a physical key on the keyboard, not to an on-screen graphical button intended for mouse operation.
