How much does does this save? Is it really worth the complexity?
It is a strong statement that React cares strongly about my page weight, even as small as 3KB. At least the implied message it sends me:
1. I really should care about my page weight too.
2. React must really need every KB it currently uses. And if not, it will soon optimize away the bloat.
With the prevalence of npm and mini-libraries I'd be interested to see if the size of React could be cut down and put into optional components.
It's not a magic bullet, but it can go a long way in helping remove a ton of unused-for-your-use-case code in your production build.
For tree shaking to work, it must be possible to statically infer that some code is not part of the dependency graph of your application. Something like SVG support requires extensions to the virtual dom node structure that the renderer's visitor looks at, so it's quite difficult to make it into a hook and not lose performance to various overheads. It's also worth mentioning that over-modularizing can get boilerplatey (e.g. https://github.com/paldepind/snabbdom#inline-example)
There is lower-hanging, bigger-buck-for-your-bang, fruit if you're trying to fight page bloat.