Three years ago, Windows XP was considered off-topic.
I initially agreed with the close votes, but, now that I think about it, XP should probably be on-topic now. Here are some reasons it was originally considered off-topic:
It's still got 5+% of the desktop market share. – Mark's answer
Down to 2% now, according to that link. According to this link, it's down to <0.7% as of last month.
Mainly because XP marks the edge of its technology type which is still common with that of Windows 7. Games and applications which run fine on XP will still work correctly on 7-10 in most cases. – Rowan Hawkins's answer
I don't think this has changed. A little more compatibility's been broken, but not all that much.
Another thing which allowed Pre-XP OS's to reach "Retro" status earlier is the fact that there isn't a whole bunch of information on the Web currently for those operating systems. Where as XP and its newer iterations have existed in the time when the Web was prolific. As a result there are still numerous websites with current information reflecting current Hardware and current software. – Also Rowan Hawkins's answer
This has changed. There's a lot less up-to-date XP information, and Microsoft's broken all the XP support links (twice over).
Win XP still receives updates till 2019 – Anixx's answer
This is the past. To paraphrase d2b's answer, Windows XP will be 19 in a few months. Its final release was 12 years ago, mainstream support ended over 11 years ago, extended support ended over 6 years ago, and the final updates for embedded versions (which you could mostly kludge into user versions of the OS via a Windows Update registry hack) ended over a year ago.
Its patents have started expiring – all the patents from the release of XP will have expired in a year and a month.
Your question wasn't tackling an aspect of XP shared with more modern operating systems – it's not about the technology shared by Vista, 7, etc.. As such, I think it's on-topic, regardless of whether XP proper is.