Bulgaria’s Defence Ministry held a procurement procedure from September 2020 to July 2022 for a company to repair and maintain the S-300 systems.
Only one candidate was admitted, Almaz-Antey Aerospace Defense Concern, which is owned by the Russian state. However, it turned out that it did not have the ability to keep the missile systems in working order.
“Unserviceable” in this case doesn’t mean the missile systems aren’t working, but that future maintenance is going to be a problem because Bulgaria doesn’t have the technicians and supplies to keep them in working order up to military standards.
So yes, Ukraine would be getting functional S-300 systems.