The dam structure is intact - the road bridge section atop of the dam was blown up.
Photos from the day of the pull out
By now the rest of the road canvas could be down, but the dam itself wasn't breached (at this time).
Nikolaev was in range of Russian artillery for more than half a year and wasn't evacuated.
OPSEC is the most likely reason - there could be some Russian loyalists/spies left behind who could provide intel on troop and equipment placement.
Oh, okay, I thought I read about blowing up the dam in one of the Russian channels so assumed it's confirmed.