In theory, yes. People have had some of their books become unavailable due to things like disputes with publishing houses/companies, for example, and books do get updated (check the Manage Content & Devices section on Amazon and see if any of your book titles have "Update available" next to them; many of mine do), so yes, in theory, a book could be edited ten years from now to remove content that was seen as okay today but not by the standards of the day in a decade.

So, I'm curious whether Amazon Kindle guarantees us that the ebook I purchased today will always stay the same as it is today.

No, there is no guarantee that the book as you bought it today will be available in the same format tomorrow or 10 years from now.

As to how to avoid it? The best way I personally know of is to download your books and store them in an app like Calibre. I buy all of my books and strip them all of DRM for exactly this reason: because I bought them, I want to keep them, and I don't want to lose access to them one day because Amazon is having an argument with the publishing company or something.