Woody, I think your idea for a taxonomy map of Dyalog and how it relates to other APLs and/or provides a view of the "evolution" of a specific APL is A Good Idea and we will try to make it real.
However, I do not personally find that starting from a historical perspective is very useful when teaching APL. I also start around Iota and adding vectors to vectors, but I try to introduce it as if "this is just how it is" - almost as if it's the natural state of the world; there has always been APL, that's just how it is.
Another thing that I am trying to do in the introductory materials I am writing is introduce the concepts in the contexts in which they are useful, as opposed to in the order in which they were introduced to the language. The reason why these developments were made in Dyalog is because there was a use case for them, or they improved some aspect of the language.
I'll give 2 examples.
In the case of the rank operator, it is an incredibly useful generalisation of scalar extension. In contrast, bracket-axis is anomalous syntax and has a different definition depending on which of the limited primitives it applies to. It also does implicit transposition in some cases which is also a bit difficult to grok. See the webinar on The Rank Operator and Dyadic Transpose for more on that:
https://dyalog.tv/Webinar/?v=zBqdeDJPPRc.
In the case of function trains, there is some debate as to whose purpose the extensions serve. Arguably, the language implementors who had the most interest in and use for them were also those with the power to have them implemented (I personally like them but I throw this point in for added nuance). I find that trains of up to around 5(+,-)2* carriages can be a useful way to extend your APL vocabulary in a way that you couldn't previously - you can write custom functions that resemble the core language in some sense, where you are generally unable to define your own primitive symbols.
While it is important and useful to include reference to APL history as both a learning aid (connecting concepts to stories and lineage can help with retention and improve understanding) and for the value it has in its own right, I wouldn't personally use it as the road map for introducing APL concepts to newcomers. Of course, this is just my personal opinion.
Having said all that, I probably would show it to newcomers at some point - just not the very first thing.
Cheers,
Rich
*in some cases, I use something like (+,[0.5]-) to add a new axis to keep the terms separate. Sometimes I wish one of comma or comma-bar would do that by itself, so I could write 1 2 3(+⍪-)4 5 6 and get a 2 row matrix with the added terms on top and the subtracted terms on bottom.