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).