The Proto-Slavic language existed until around AD 500. By the 7th century, it had broken apart into large dialectal zones.
There are no reliable hypotheses about the nature of the subsequent breakups of West and South Slavic. East Slavic is generally thought to converge to one Old East Slavic language, which existed until at least the 12th century.
Zaliznyak and Nikolaev's points mean that there was a convergence stage before the divergence or simultaneously, which was not taken into consideration by Kassian-Dybo's research.
This general process of palatalization did not occur in Serbo-Croatian and Slovenian. As a result, the modern consonant inventory of these languages is nearly identical to the Late Common Slavic inventory.
The modern languages vary greatly in the extent to which they preserve this system. On one extreme, Serbo-Croatian preserves the system nearly unchanged (even more so in the conservative Chakavian dialect); on the other, Macedonian has basically lost the system in its entirety. Between them are found numerous variations:
The following is a very brief selection of cognates in basic vocabulary across the Slavic language family, which may serve to give an idea of the sound changes involved. This is not a list of translations: cognates have a common origin, but their meaning may be shifted and loanwords may have replaced them.