Why can’t I use my back camera on portrait mode on iPad 2019?

No. This is a hardware issue. That means that it is a device issue, and you have to wait for the next edition to see if it supports rear camera portrait mode. You can just edit it, and that is my suggestion. There is no fix to the camera for it to work.

Portrait Mode only exists on iPhone and iPad Pro.

You just simply can't. It isn't like you can't fix it. It was made without portrait mode on the back camera. You could edit it, though.

You cannot fix what you do not have. There is no hardware to provide what you are seeking.

Portrait mode works for Selfies only.

Take photos with the camera on iPad - Apple Support

Precisely. You can only use it on the front camera. Like you said, there will be no fix for this until they add this feature in an upcoming iPad. They might not ever add it.

So unless you are taking a Selfie the PORTRAIT mode is useless? And there will be no fix for this until a newer version of the IPad is released?