Should I sell away my STI ETF and when should I sell? Is a <20% loss okay?

The advice I tend to give regarding cutting losses is generally that, if you have a better investment idea to put your money in, it is not wrong to cut your loss and move to another investment.

So right now, what I would do is to compare your expectations for the STI in the next few months to the next investment that you'll likely shift your funds to if you were to sell off your STI ETF investment. If the prospects on the other investment looks better, go ahead and shift it over. There is no shame in cutting a loss.

Personally, I am not vested in the STI.