From the announcement post:

Hey guys! So, on November 25th, at 3 PM UTC, Sin Slayers: Reign of The 8th (no GOG page yet) will be released on our platform. Having said that, the publisher decided that with it, the "original" version will be no longer available to buy, which is:

Sin Slayers + all its DLCs + The First Sin (the game's demo).

The removal will happen on the same date as the release of Reign of The 8th. If someone would like to grab this original version (the new version will have some new enemies, but also changes to the game's flow, balance, and optimization) then please do so before the release happens.