“Kyiv and US observers point out that the Kremlin has failed to take it for more than a decade”
A part of the contested area was controlled by separatists early on, but they soon lost area.
WIKI
Sloviansk was one of the focal points in the early stages of the war in Donbas, in 2014, as it was one of the first cities to be seized and controlled by Russian-backed rebels (separatists), in mid-April 2014. Ukrainian forces then retook control of the city in July 2014, and since then, Sloviansk has been under Ukrainian control.
The 2001 population of Sloviansk was 141,066. Largely due to the ongoing war in Donbas, by early 2022 this was down to 105,141. Following the 2022 Russian invasion of Ukraine, the city’s population fell markedly, to around 24,000 by July 2022, due to shelling, and ongoing war, according to Ukrainian authorities. In April 2023, The Financial Times estimated the population to have recovered to 40-50,000.
https://en.wikipedia.org/wiki/Sloviansk
Putin’s best General Prigozhin would have captured Sloviansk but Putin decided to blow him up instead.