The actual ending of Slay the Spire is now available, an unlockable 4th act. It is not much of a 4th act, but it gives you a final boss fight for the game. I don’t think it adds much except for a sense of finality. Except that you can immediately start another run, so final isn’t final.
Slay the Spire has clearly been my game of the year, providing me with more playtime than most MMOs I have tried. It’s great, worth buying even without a sale. It feels a little anticlimactic that adding the final boss seems less significant than some of the regular weekly updates. The baseline for the game sets the bar pretty high.