Do decaps only count when it happens in official servers?
And, do they count even if you lose?

Win or lose the match, it is still a decap. And it counts everywhere, not only in official servers.

So, it counts?
How about it happening in unofficial servers, still counts?

Yeah they all count no matter what room you're in, and no matter what mod you're in.

But if you are trying to get a decap achievement they are currently broken at the moment so they don't count.

Oh, that explains why I haven't been getting achievements, I got more than 20 decaps, but it doesn't count.
Thanks.