The settings and themes in Stephen King’s stories) and real-life elements that are quite obscure, such as Project MKUltra. With the story set in the 1980s, the writers have had plenty of creative freedom with some of the most bizarre and dangerous things that took place during the Cold War and the paranoia that came with it.
Stranger Things season 1 was set in 1983 and focused on the disappearance of Upside Down. Season 2 caught up with the characters a year later and dealt with the aftermath of Will’s time at the Upside Down and the arrival of a new creature from the strange and deadly dimension. Season 3 was set in the summer of 1985 and showed that the Russians have been hiding and experimenting in Hawkins, thus leaving the door open to all types of dangers. Supposing season 4 follows the pattern and is set in 1986, it could easily connect to the worst nuclear disaster in history.
Mind Flayer are up to the imagination.
Another way Stranger Things can connect to the Chernobyl disaster is by using it as cover-up. Again, one of the Russians’ experiments goes wrong (whether they release a creature worse than the Mind Flayer or create one. There are many ways it can go wrong), but as these are secret operations, they will need something to divert attention – enter the Chernobyl accident. This would serve as a screen for them to fix whatever happens with the Upside Down and what comes from it, without the rest of the world knowing that these experiments are happening.
Chernobyl received renewed attention this year thanks to HBO's critically-acclaimed miniseries Chernobyl, which delved into the horror and tragedy of the accident, and its place in Soviet history. It was still fresh in people's minds when Stranger Things season 3 released a month later, so many fans made the connection between the Russian presence and the time period, theorizing that Chernobyl could play a role in the next season of the show.
There are no plot details yet for Stranger Things season 4 and it’s possible that the writers will choose to skip the Chernobyl disaster and not use it at all (maybe not even acknowledge it), but the Upside Down being somehow related to the nuclear accident would not only make sense and fit with the themes of the series, but it would also bring a lot of creative possibilities that would expand the series' mythology.