I am not sure whether or not older versions of Mac OS X are classified as retro. I am talking about versions such as 10.0 - 10.6.
In my personal opinion, I would draw the line at the PPC/x86 changeover. Others are, of course, free to disagree and discuss. To me PPC Macs running the earlier OSX versions fall into the same category as Amigas with PPC accelerators, PPC BeOS machines, etc, all of which I would consider part of the retro scene, as they go beyond simply being obsolete and extend into the realm of requiring a separate skill-set to maintain and use that an SE dedicated to say, modern Macs, might not be able to provide adequate answers for.
Really, that's the ultimate decider. Will you get good answers on another SE, or is the topic simply too obscure via the passage of time (retro)? Often, a search of the modern SEs for similar questions can give you a good idea if you need to bring it here to find the knowledge-base to answer your question or not.
Without having seen any questions, I would speculate the answer is no — not on topic.
There are several discussions describing the scope and purpose of this site, but I will refer you to one of the answers I posted here: What constitutes retro?
I defined retro as computing systems which are
…not considered contemporary to what is modernly available.
While OS X has certainly advanced over the years (and some featured/methods deprecated), I wouldn't put "older" OS X questions in the same space as the antiquated, specialized interests of this site. Questions here are those I would consider "a lost art" of sorts; which I aptly try to describe as
vintage-computer hobbyists interested in restoring, preserving, and using the classic computer and gaming systems of yesteryear.
I'm not sure I would put OS X in that same mold.
The specialized interests driving this site don't generally include hardware, software, or systems which simply passed the sunset of support. Some technologies advance incrementally… but what comes next tends to follow the basic precepts of what came before — that is what consider widely contemporary to what is available today.
Sometimes technologies have a big breakthrough or break off from how things were done before — things change forever, usually for the better — but people essentially "forget" it was ever done another way before and it takes a specialized interest to support it.
That's where this site comes in.
I would invite further opinions from folks closer to this subject, but I do not believe OS X questions fit the spirit of this site.