It started on just niche devices like the TiVo. It's been slowly branching out to more and more devices since then. Also, have you never seen an Android device where sideloading was disabled and you couldn't re-enable it? Because I have.

Then kindly reveal which Android devices are this crippled - not being able to install software the normal way, via ADB - so we can avoid them.

I also wonder: how can you develop on them? Android Studio would fail app deployment, the way you tell it.

You don't develop on them. That's the point.

One device I know of that doesn't allow sideloading or developer mode is the Ratta Supernote A5X/A6X.

Is it really expanding though? The market for devices is also expanding. Don't confuse absolute numbers with relative numbers. Of course I have encountered devices that don't allow sideloading, are you saying that is expanding also? Or has "rooting" always been something many carriers and manufacturers sought to disable?

In both absolute and relative terms. Basically every new computing device that's not a desktop or laptop is locked down in some way, and even some laptops are locked down too (e.g., Windows RT on ARM laptops).

All Android devices are locked down? That's not true, most Android devices can be "rooted" quite easily.