Unless you can also downgrade, “opt-in” is a misnomer IMHO.

Absolutely. This is the "ratchet effect" which is everywhere in software these days. The best you can do is defer clicking the ratchet forward for a while. You can't avoid it forever, and you can't ever turn it backwards.

Many major operating system updates to computers of mobile phones are also "opt in" while not providing a way of downgrading—granted in the case of computers you can often "just" reinstall from scratch, possibly factory reset in many mobile phones has achieve the same?

Granted Tesla is pretty naggy about the updates should you choose to hold off. But as far as I know, it's not limiting any functionality should you choose to do that.

For example the Polar watch app simply says you need to update the app firmware to keep using the app: that's not opt-in.

How? You’re not forced to, are you?[a] If you can research the changes and decide to hold off, it’s still opt-in.

[a]: Genuinely asking. I don’t own a Tesla

It nags you incredibly. Everytime you sit in the car, the pop-up is there, nagging to install now, or to schedule install overnight. The way out of the pop-up is a small [x] to close it. And the dialog is right back the next time, waiting until you (or somebody) will accidentally press the other button.

I have it sitting on that screen, nagging me, and, honestly, it's depressing.

I do own a Tesla.

You're not forced to. The car will alert you when there's new software available and even download it when you're connected to wifi (which I never am because I live in an apt and park far from my actual apt), but you still need to decide when to schedule the install, because it can take 30-90 minutes to complete. The UX has not yet degraded to the point where Tesla decides 'eh, you don't need your car at 3am to go to the airport/hospital today.'

I do believe there are cases with critical bugs where Tesla can force the update.

I have the v11 update sitting on my car and I'm not installing it until I hear some of this stuff is fixed.

Because a lot of times you want to actually use the new thing for a while to see if it works for you. But with no way to go back, it's a one way door, so you better have access to someone else's Tesla if you want to just try it.

A one way door is still opt in. Is it scummy? Sure. But is it not opt in? No. We can argue points without hyperbole.

When the only way to fully understand the result of going through the door is irreversibly going through the door and hoping for the best, it is not opt-in. You do not have an opt, you have a guess.

Perfectly put.

See P.T. Barnum's, "This Way to the Egress" http://www.ptbarnum.org/egress.html

The problem is that it is all or nothing. You can't accept each new feature separately only all of them at once.

Regardless of the platform and device, updates are opt-in until a critical feature requires the latest version to even work (effectively deprecating the older versions).

That's just untrue.

Telsa will eventually push an update through to your car over cellular. (or during service, when your car will automatically connect to the tesla service center wifi)