While we’re not sure it’s intended, the Brawler’s Guild is currently available on live servers.
According to player reports, it’s possible to queue for the Brawler’s Guild bosses on live servers. While no new challenges have been added, the encounters have been properly scaled for Dragonflight (Level 70).
I went to Orgrimmar on live servers and can confirm the Brawler’s Guild is currently open, at least for Horde players with all the BfA bosses.
The Brawler’s Guild has been temporarily disabled in the Shadowlands pre-patch on October 13, 2020.