The description should still be there in the Repository Settings (under Repository Details), however it's no longer displayed on your repository overview page.
Instead, Bitbucket encourages you to create a README.md file in the root of your repository which will be displayed on your repository overview page. Markdown is much more customizable than just plain text, but plain text is fine if you just want to copy your existing description over. The easiest way is via the online editor, which you'll get if you click the "Create a README" button on the repository overview page.
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