Today on 9/9/2019, Celeste is available for $9.99 and the free DLC for Chapter 9 has also been released. This current price on the NA Nintendo Switch eShop is 50% off the full retail price the discount will last until September 16, 2019.
Ch 9 Farewell includes new levels, new mechanics, and new music. It is recommended that you complete the C-Sides as the new chapter will have a similar difficulty. The free DLC is available on all platforms with Celeste and is also 50% off on Steam.