The WHO wrote a report in 2006 and stated that the incidence of thyroid cancer, which was the main concern, had dropped to near background levels with only a small increase in pre-menopausal breast cancer in those most exposed. Regarding the population with lower exposure:
Projections concerning cancer deaths among the five million residents of areas with radioactive caesium deposition of 37 kBq/m2 in Belarus, the Russian Federation and Ukraine are much less certain because they are exposed to doses slightly above natural background radiation levels. Predictions, generally based on the LNT model, suggest that up to 5 000 additional cancer deaths may occur in this population from radiation exposure, or about 0.6% of the cancer deaths expected in this population due to other causes. Again, these numbers only provide an indication of the likely impact of the accident because of the important uncertainties listed above.
A more recent report in 2011 from the NIH looked at Chenobyl residents who received higher exposure and is not so welcome.
The researchers found no evidence, during the study time period, to indicate that the increased cancer risk to those who lived in the area at the time of the accident is decreasing over time. However, a separate, previous analysis of atomic bomb survivors and medically irradiated individuals found cancer risk began to decline about 30 years after exposure, but was still elevated 40 years later. The researchers believe that continued follow-up of the participants in the current study will be necessary to determine when an eventual decline in risk is likely to occur.
So, it seems prudent to keep monitoring all those affected survivors as the risk may extend past 40 years. Presumably more data will come to light with time.
Note that the I-131 risk was thought to be mainly from drinking cows milk with cows feeding on I-131 contaminated pasture, and concentrating it into their milk.