Some background, skip it if you know it already:
According to Kant, all human beings work like this:
1. There are two types of phenomena: space and time. These are like they are in themselves, and we cannot know them. The eye sees them but cannot understand them, so we cannot know them.
2. These data from the senses will be passed to the categories or concepts. This happens a posteriori. The concepts form the "administration system" of the soul.
3. After combining the phenomena and the concepts, we can know the things. However, because of step 2, we do not know the pure phenomena. We only know how they look for us.
As a side note, this is why he says:
"Thoughts without content are empty, intuitions without concepts are blind."
- Immanuel Kant
You are correct that this means that when you remove the subject, there is no knowable reality anymore. The phenomena might exist, but that subject that has been removed cannot know it, and since he is removed, he cannot know the things like they are to him either.
You have no physical information from the senses about history: you have never been there. That means the image you have of history or the world without humans is purely based on the world as it is now (not as it has been) and fiction. You only have read history books, seen pictures, etc., and all that makes you able to imagine a world like it is described. However, that image of the world does not come from the senses at all. It's fantasy.
The same goes for imagining a world without humans. That isn't based on information from the senses but on fantasy.
This is how these worlds seem to exist in your mind, however they did not come from the senses. Imagining a world without minds is no objection against mind dependent reality because in that case you'd still need your own mind to imagine it. The point Kant makes is that you cannot imagine such a world with knowledge.
Now on the body of your question, which seems to ask a different question.
Kant indeed means that when a subject is no subject anymore, his world vanishes. So the phenomena don't vanish, but the concepts vanish, so that he cannot know the world anymore.
I'm not sure about if Kant thought there would be an afterlife, so I cannot say if this would happen when someone dies.