It was never really part of the OS, it was always an additional package on top of the OS. I say that as someone who has been running OS3.9 without Reaction on 68000 systems for more than a decade. And from what I have read so far, even 3.2 will work perfectly fine without Reaction classes. Reaction is what people find “bloated” and “slow” with OS 3.5/3.9.
Err, what? So how is the palette preferences going to work without the color wheel gadget and the gradient slider gadget? How is the datatypes system to work without them? How is the icon editor to work? Just in case you confuse something: "Reaction" is not a "new subsystem" of the operating system at all, it is not isolated, and you cannot draw a line between "here reaction starts and here the os ends". It is all the same system to begin with, and "Reaction" does not invent anything anew. It is called "boopsis" and is part of intuition.
That is quite different from MUI, for example, which has its own system library, which loads its own components, and builds its own GUI based on its own logic and its own dispatchers. MUI reinvents the wheel. Boopsis don't.
What we call "Reaction" these days are just a collection of boopsi classes that use the intuition boopsi interface along with the exec library interface. There is no technical difference between the boopsis that came into the Os at CBM times, and those that arrived later and were implemented by Chris Aldi. Quite unlike MUI, it does not reinvent anything at Os level, just uses all the interfaces that are already present.
Why you want to draw a line between the original CBM boopsis and the boopsis provided by Chris (originally) and which were then further developed by multiple parties is beyond me.
You may want to argue that boopsis in general are too heavy weight for low end machines, mostly due to their "smalltalk" like dispatcher mechansim which goes through many indirection levels. That is certainly true, and one of the reasons why the GUI for 3.2 is based on an extended (scalable) gadtools version that uses boopsis only occasionally, but it still means that this gadtools interface uses boopsis (or "reaction" - there is not really a difference despite the name) after all where practible. In fact, as I already mentioned, this happened in 3.1 already. The colorwheel, the gradient slider, even the sketch pad of the icon editor and the pointer preferences: All these are boopsis. Or "reaction". No matter what you name them, it's all the same.
I would be just mad to ignore such contributions and leave them aside just for the name part, which is - as said - purely artificial.