Blizzard has confirmed that Hardcore servers will not advance into TBC Classic. Here is the blue post from Community Manager, Kaivax:
Anniversary Hardcore Realms
As there are no Burning Crusade Hardcore realms, during the transfer window, players with characters on Anniversary Hardcore realms will have the option to transfer their characters to Normal (non-Hardcore) Anniversary realms, so that those characters will continue to Burning Crusade Classic Anniversary.
Players who do not move their Anniversary Hardcore characters to Normal Anniversary realms before January 12, 2026 will stay on their current realms, and the transfer destinations available to these characters will change to non-Hardcore Classic Era realm destinations. This means that an Anniversary Hardcore character who meets their untimely end will be able to transfer to a Classic Era realm and return to the living.