TrueCraft is an entirely clean implementation of Minecraft beta 1.7.3 from September 2011 without any decompiled code.
As you know, Minecraft was originally a simple, indie game that blew up beyond what anyone could have imagined and eventually acquired by Microsoft. Over the years so much has been added and there's even a "Story Mode" version that attempts to add a storyline to Minecraft.
Somewhere around version 1.73 is when most people who loved the original feel that the extra fluff and over-the-top things that were added took away from that indie feel.
TrueCraft is intended to be a more basic Minecraft before things like the End, all-in-one Redstone devices, or village gift shops appeared. So, it should play like a simple sandbox game.