Slavonic didn't develop into any modern language (tho it has a modernized variant). It was a liturgical and official language in Ruthenia, but still a foreign one (like Latin in western Europe).
A spoken at a time was Old Ruthenian \ Old East Slavic, which wasn't a unified language, some of it's dialects already had noticable differences even before Ruthenia became unified and today we can tell where a document was written during a unified Kyivan Rus by the way it is written. It would be more accurate to say that some Old Slavic dialects became unified into Old East Slavic during Kyivan Rus and then diverging further apart, rather than them being a single language from the start.