If you read the book Voice of Chernobyl (on which the TV series was based), by about half way through it becomes fairly obvious that a lot of the liquidators must have died of radiation related diseases after their service, just from references to "he was the last one of his group to die" etc. However there is a scientific literature, mostly derived from a single Russian Scientist, V. K. Ivanov and his team, that actively works on denying this. The biggest single indicator that something terrible happened is simply the life expectancy chart for Russia which shows a profound dip for about 5 years after Chernobyl. (This gets blamed on soaring alcoholism post collapse, which isn't particularly credible.) Generally I would treat claims that there was little contamination, and that contamination didn't spread, with scepticism. There was a lot of looting of the closed off areas.
Meanwhile the liquidators themselves are starting to get organised and their estimate is about 200,000 early deaths out of 600,000. Which does line up with the life expectancy chart.
https://www.abc.net.au/news/2019-06-22/chernobyl-what-it-was...
The most recent documentary on Chernobyl using Russian archive footage has a few minutes hinting at significant mutations in children at at the time, but it's hard to know what the details are behind that.