Genuine question: if your Apple account is locked, and you're unable to create a new one, is your iPhone still usable?
This is one of the reasons the used market for Apple devices is absolutely fraught with danger. If an Apple ID is left active on the device, only Apple can reset it. In most cases, they will only do that if they are provided the original purchase receipt for the serial number associated with the device. So in theory, removing the activation lock from owned devices is possible in a situation where a locked apple ID cannot be recovered if you are the original owner. IMO, there should be a process to release devices that haven't been used for a certain amount of time AND haven't been reported stolen. But there's very little incentive for Apple to do this.
If you read the other posts about this, the author explains that the phone technically still works, but you can't access iMessage or anything. Probably basic text and calls only.
Yes, you can continue to use anything that doesn’t require using Apple services.
So you could use your existing apps but not download new ones from the App Store.
You could use iMessage with some restrictions. You could use Apple Music but only the free radios. You could use Apple’s photos but would lose sync.
Usability depends on how much you rely on those services, but the device itself is still useable for other things.
Not really. I have an iPad without an Apple account and you can’t do much with it.
That said, I choose to use it this way and it does everything I need it to.
In a genuine and everyday real sense, no, your likely thousand dollar device is not usable. The App Store requires an account to download from. Internal services and apps often complain about not being available. You are mostly stuck with whatever built in, non-cloud services the device comes with, which isn't much. Weather and mail fetching come to mind. Maybe some of the simple recording / note taking like apps. A working Apple ID is essentially a requirement to actually use the device you purchase. And yes there will be comments from folks about "ways" you can perhaps sideload or get things running, but to a regular person that simply uses a phone like a standard appliance in their life - they're stuck.