I’m no loner active maintainer of zimlib but I’m just talking from pacakaging convention followed by Debian - a library is named based on lib<library name><major version> where major version is major part of soname. And that is precisely followed here libzim7 indicating 7 as major release number.
Note that this request is about changing the source package name, from "zimlib" to "libzim". The bug was accidentally filed on the binary package instead of the source package.
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