From what I remember, if a dev decides to remove a game from the website, it will not be available for download. Devs are discouraged from doing that, but at the end of the day, it’s their decision to make.
I would suggest to download a game as soon as you buy it, and keep a local copy of it. This will guarantee that you will have access to it forever.
Hope that helps :)
Edit: Beaten by 7 seconds, will keep this comment up for completeness.