It's easy in gcc. You simply say:
#warning FIXME: Code removed because...
So that's me sorted for Linux and MacOS. I'm happy in the Fun Place.
But in the Dark Place I was clueless. How do you say #warning to Visual Studio? You can happily write #warning in C#, but not C++.
Interestingly, the answer fell below my Google/Boredom Threshold (i.e. a web search didn't reveal the answer in sufficient few clicks that I lost interest). I just shoved in a run-time assertion instead. It'd do the job, but not as immediately as I would have liked.
Thanks to hashpling and the miracle that is Twitter, I now know the answer, and share it with you in the vein hope it might come higher up the Google rankings for those poor souls that follow me:
#pragma message ("FIXME: Code removed because...")
Needless to say, this is all tediously non-standard.
For bonus points
This still doesn't get us exactly the same behaviour as gcc's #warning. The message is produced, but without file and line information. This means that if you double-click the message in the VS IDE it will not jump to the warning in the editor window. It also means that build logs aren't much use.
Sam Saariste pointed this out, and here's the standard preprocessor mumbo-jumbo you have to jump through to get the exact warning behaviour I was after:
Couldn't be simpler, could it?!