I had the opportunity to talk with city officials this week and asked the question.

They confirmed this building is still considered the official city hall; the Mayor still has a ceremonial office there.  It is also referred as such on their official website (https://www.brussel.be/stadhuis – fun fact, it is now open for visitors).

All the administrative services have relocated to Brucity, and so have the Mayor and the eldermen, but Brucity is some sort of headquarters but should not be considered as the city hall.