But it’s never stopped meaning that, though? It’s still extremely commonly used to tag that sort of content, and I don’t know why you’re starting to insist that words don’t mean the things they say.

The literal meaning of the phrase is useless, since what is safe at a workplace varies drastically with the workplace. It's also relevant in a lot of other settings that aren't work, like browsing your phone on public transport or at school. It's also very common for phrases to have a different meaning from the literal meaning of the individual words. I'd say that's even the case for most phrases. When you say it's raining cats and dogs there aren't actually cats and dogs falling from the sky. In this case the actual "work" aspect isn't relevant.

What the tag does signify is that the following content might be disturbing or inappropriate in certain situations so you can be aware before opening it. And bikini pictures don't warrant that warning, even if some workplaces have a policy against it.