Do you mean Intel x86 processors from the 8-bit to 64 bit versions? Or do you mean PCs based on these running Microsoft DOS and Windows, which is a much broader topic?
For just the processors, I'd say anything that went obsolete long enough ago to now have some nostalgic interest is on topic.
So 8080, 8086, 8088, and 286 are definitely on-topic. 386 is probably on-topic considering that even the Linux kernel just dropped support for it. There's surely a range of old stuff that worked on 386 systems that doesn't work on modern systems. These days I think a decent argument can be made through 486 and Pentium. Maybe not for all 32-bit stuff though since I think 32-bit stuff is still being made. One of my netbooks is 32-bit but doesn't seem at all retro. Anything 64-bit can't be retro yet, even though it's been around for years now.
Stuff like Real Mode and Protected Mode and segmented memory models and DOS extenders are surely on topic.
Any OSes that don't run on current systems are probably on-topic. Real DOS is probably always on-topic. That doesn't mean all Windows console stuff is on-topic. People probably still confuse the two. OS/2 is probably on topic, unless it's still made?
Stuff about old versions of Windows prior to Windows 2000 is surely on topic since that's when all Windows versions were based on Windows NT and the versions based on DOS became obsolete.
Look, "retro" in general means that it went old and boring and after some years became interesting again to a certain community, generally out of nostalgia. So if it's clearly still just "old and boring" it has no place here. If it did its time in the "old and boring" phase and then regained interest, then it's on-topic. There will be a blurry line between that might warrant discussion sometimes.