Is the usage of sago in food items prohibited in Islam?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Sago is a starch extracted from the spongy centre, or pith, of various tropical palm stems, especially those of Metroxylon sagu.  It only contains small amounts of protein, fat, and fibre and lacks many vitamins and minerals.
Starches are complex carbs that consist of many connected glucose molecules. Glucose is a type of sugar that your body uses as an energy source. 
According to the above information, there is no prohibition of using Sago as a food ingredient.
And Allah Ta’āla Knows Best
Mufti Muhammad I.V Patel
Checked and Approved by
Mufti Nabeel Valli
Darul Iftaa Mahmudiyyah
Halal Food Production- Page 342