There's a closed pull request on the void-packages repository

obnam: remove
https://github.com/void-linux/void-packages/pull/14467

Description:
It was retired over two years ago: https://blog.liw.fi/posts/2017/08/13/retiring_obnam/

On the home page:

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The Obnam project is retired. See https://blog.liw.fi/posts/2017/08/13/retiring_obnam/ for more information. Please use another backup solution instead.

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It is also python2 (EOL in 2020).

Nothing depends on it so it is OK for removal.