Day of the Dead is a 1985 American zombie horror film written and
directed by George A. Romero.
AJ's answer is pretty spot on in terms of overall theme, but there are couple of plot points to add that are in parallel, including Day of the Dead is also a ZOMBIE film and the Mind Flayer was taking others like Billy, Heather, Tom, etc and using them to get things done...
Mild Spoilers About Billy
He then ran out of the mill and got back into his car, speeding away
in a panic. He stopped at a payphone to call 911, but as soon as the
phone picked up, he finds himself in the Upside Down, and a group of
people emerging from the fog, slowly walking towards him. He was
approached by a copy of himself, and asked what it wanted. It told him
to "build what you see", referring to the zombie-like mob behind it,
then suddenly he returned to the real world, clueless as to what
The Mind Flayer's over all goal was/is to destroy El, her friends, and then the world! Day of the Dead poses the same apocalyptic zombie threat, which in theory, may be more relevant to Stranger Things season 4.
In addition, even though they weren't "zombies", Steve and Robin get into a situation where they are heavily drugged, and ultimately not acting like themselves for a couple of episodes. And the Russian assassin character, Grigori, may have been a reference to an evil Terminator featured in Terminator 2: Judgement Day, who would stop at nothing to accomplish his goal. To clarify these two instances are depictions of these characters being somewhat "mindless" in nature, like zombies.
In an underground facility in the Everglades housing scientists and
soldiers, the scientists are trying to find a solution to the zombie
pandemic, in exchange for the soldiers' protection.
There is also the prospect that Day of Dead featured scenes with scientists in underground military bunkers that broadly parallel itself with Stranger Things scientists and more specifically, The Russian's underground facility in season 3.