The Second Battle Cruiser Division is a unit within the Alliance Fleet. At the time of the flight from the Syndic home system, the Second Battle Cruiser Division consisted of the battle cruisers Leviathan, Dragon, Steadfast, and Valiant. The Division is commanded by Captain Tulev of the Leviathan.

In what appears to be an error in the text of book five, The Lost Fleet: Relentless, the battle cruiser Steadfast was apparently destroyed during the battle in Heradao star system:

The four Syndic battleships anchoring that corner threw out a barrage of hell-lance fire that tore apart Steadfast, riddled Intrepid, and hammered Inspire, while Illustrious took more damage on top of that suffered at Cavalos, and Courageous spun out of control as the Alliance warships cleared the Syndic formation.

Intrepid thinks she can keep up but all of her combat systems are out,” Dauntless’s combat watch reported. “Inspire has full maneuvering but has sustained heavy damage to weapons systems. We can see escape pods leaving what’s left of Steadfast.”
"Tore apart" plainly means the ship was broken into large chunks, and the phrase "what's left of Steadfast" certainly implies it's no longer whole. Alliance sailors, brave souls that they are, also tend not to abandon anything other than wrecks.
However, in the next chapter, Steadfast is not listed amongst the lost ships:
THE butcher’s bill after a battle was always the worst part. Geary read through the names. Courageous, Intrepid, Exemplar, Goblin, heavy cruisers Tortoise, Breech, Kurtani, Tarian, and Nodowa. Light cruisers Kissaki, Crest, Trunnion, Inquarto, and Septime. Destroyers Barb, Yatagan, Lunge, Arabas, Kururi, Shail, Chamber, Bayonet, and Tomahawk.
And there is no mention of repairs to Steadfast in the scenes post-combat or, indeed, the rest of book five or book six, The Lost Fleet: Victorious.

Despite this, in the subsequent books' opening Ships of the Alliance Fleet sections, Steadfast remains amongst the other ships of the Second Battle Cruiser Division, and is mentioned in the text of Beyond the Frontier: Dreadnaught, the first book of the sequel series.