It's time to seriously think about this. Windows XP is almost 15 years old, and is the last operating system to run pretty much any 9x software.
Sure it uses the NT kernel, but it's so old that it doesn't really matter (like 2000)
It's still got 5+% of the desktop market share. Regardless of how old it is, I think that pretty well disqualifies it.
I would say not yet. 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. That is not the case for WinME and its predecessors. This is especially true if they do any type of direct hardware access which precludes XP or NT's HAL.
I would hazard that XP would need to be in End of Life status for at least 6 years before anything could be considered retro.
Edit: 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.
Windows XP as a whole? Certainly not yet. But 16-bit subsystems of Windows are strongly connected to the topic, and retrocomputing.SX should encourage questions about running 16-bit software on Windows (XP included). Therefore “doing under XP” may not be considered a definite off-topic and respective tag should be created (supplied with usage guidance).
Our Help Center welcomes questions relating to
how to use or preserve computing equipment that is no longer manufactured or supported by the manufacturer.
It later clarifies
Questions about modern, currently supported computers are off-topic. This includes questions about earlier versions of a current machine or OS.
The last vendor support for this platform ended in April 2019, so it is clearly no longer currently supported. Anyone seeking to create or resurrect a Windows XP system and requiring help will need to get it from the community (i.e. Stack Exchange) rather than from the supplier.
I interpret the Help Center advice to mean that questions about Windows XP are on-topic - unless it applies to Windows more generally (when XP can be considered merely an "earlier version" of the Windows platform).
There's a bit of interpretation needed because current Windows platforms can run many (but not all) programs written for Windows XP. However, I believe we should be able to distinguish reasonably well between on-topic and off-topic based on what I quoted above.
Windows platforms aren't interesting to me, so I've added
windows* to my ignored tags list. I encourage anyone else who doesn't want to see Windows questions to do the same (rather than have us deny answers to those who have Windows questions).
My personal opinion:
if your question about Windows XP is primarily motivated by intellectual curiosity – especially if it is a question about history, motivations behind design decisions, the development process and beta releases, obscure legacy features, etc – it is on-topic
if your question about Windows XP is because you still use Windows XP in your day-to-day life (maybe you live in Armenia) and you need help with it (e.g. suddenly it won't boot) – not on-topic
Intellectual curiosity vs pragmatic or commercial need is an important dividing line, I think. But not an absolute one:
a question about Windows 10 is off-topic, even if it is purely motivated by intellectual curiosity rather than practical need
maybe someone out there is still running their business on an Apple II, in which case I think they can ask for IT help here, even though their need is practical rather than curiosity-based
Windows 10 is new enough (and still used enough) that it is off-topic, even for curiosity-motivated questions. Apple II is old enough that it is on-topic even for practically-motivated questions. Windows XP currently occupies the grey area in-between, on-topic for the curiosity-motivated, off-topic for practically-motivated. Eventually, as its use further declines, it will move out of that grey area, and be on-topic without exception – but we still aren't there yet.