I'm not voting because I don't feel there's an answer option that properly captures my view on the matter.
I don't really find them particularly attractive or unattractive. Very severe cases aren't very attractive but they're also often painful and a true hinderance to the affected person so I have a hard time really counting it against the person and there are medical treatments available that can both alleviate the pain associated with it as well as reduce the severity of the appearance of the concave chest. And by the same token, I also find people who are particularly bulky and muscular, chest region included, less attractive.
Preferences are preferences but there's a point at which preferences just aren't that important. You may prefer the looks of a man who's 6'3 but fall in love with a man who's 5'8, and I'd hope in that case your heart would be generous enough to not just overlook the fact that they don't fit your preference but to genuinely find them attractive.
The same is true for concave chest. I dated a guy with a relatively mild one and while it was certainly noticeable, it had almost no affect on how attractive I found him and was more just a unique quirk of his body--perhaps in part because I loved him deeply. Instead of being a detractor, I found that the concave in his chest was a particularly welcoming place to rest my hand, stroke or kiss while cuddling and having sex.
If I were to build a "perfect" body, it probably would not include a concave chest (it would likely not include any physical disability or abnormality, either), but that's as much for the health and wellness of the designed individual as my own aesthetic preferences. All the same, I still remember my ex's chest when cuddling with new, non-concave chested men, and can't help but miss it with the same fondness with which I miss him.