My dad had an autoiimmune dicease he fatally dies . would the doctors have told my dad he would have died a long time before his death?

Maybe not: For specific answers to your dad's situation you would need to ask his doctors. In general there are many "autoimmune diseases" some can rapidly worsen and get very severe in a short time. This may be relatively unexpected and the doctors may not expect a sudden deterioration that causes death. In other cases the disease progresses slowly over time and in other cases it may be controlled for year.