Not sure why this was dead, or why Apple seems to get a pass on this walled-garden behavior. If Microsoft were to require Windows apps to be installed from the store, people would be outraged. When it's done on mobile (how the majority of the world accesses computing these days), it's seen as a valid "security measure".

Its pretty simple. Most users aren't hackers.

Apple doesn't make phones to be hacked or tweaked or whatever, they make them to be simple to use and as secure as possible. For most people, this is far more interesting and appealing than being able to side load apps.

Isn't the Pine Phone fundamentally a device made for hackers?

Fundamentally, the market for hacker oriented products is far smaller and less interesting than the market for iPhones. So hacker oriented tech is by nature going to lag the market or be more expensive.

It didn't go anywhere. Even in the 90s, there were all kinds of devices on which you were completely unable to run your own software.

There still are.

Nothing has fundamentally changed.

It didn’t go anywhere. PCs, Macs, and Android phones still work that way.

There is also an option for people who want a curated environment.