Can someone please tell me if Stex exchange is still viable and if so when will the exchange be operational
Sure it is

The STeX Team is actively developing the backend and is working very hard to release the alpha version in Q1/Q2 as the ICO got delayed a bit. They have given us some sneak peaks by releasing some modules that they have developed, like DEEP (
https://deep.stex.exchange/) and their BTC/ETH gateway (
https://www.youtube.com/watch?v=EK1Ub5IsfYM).
And of course, the last video: