One of the changes that went into general availability last week was the retiring of the "sr-Latn-CS" language code, Locale ID 2074.
"CS" in the sr-Latn-CS was the former country code for the State Union of Serbia and Montenegro until it was dissolved in 2006 and Serbia and Montenegro became separate countries. "RS" is the code for "Republic of Serbia" not to be confused with Republika Srpska. I have never noticed any UI difference between sr-Latn-CS and sr-Latn-RS, they were probably identical. Many users in countries other than Serbia where Serbian is used, such as Montenegro or Bosnia and Herzegovina, tended to use the code because it did not mention Serbia in the name. In case you're wondering, yes the versions of Serbian used in Bosnia and in Montenegro are different from the one used in Serbia, in fact there are a few more letters in the alphabet. However they are not supported as distinct languages in SharePoint.
The locale IDs sr-Latn-RS and sr-Latn-BA still exist in Delve, labelled with the country name for display language and for locales, and if you choose one there is a mapping done to the supported version of Serbian that uses the same script.