It seems that sometimes people mix up plot armor and being overpowered. Let us take a look at both these tropes.
Plot armor and being overpowered are different things, even though people sometimes mix them up. We will try to explain the two.
In a sense, all protagonists have plot armor because without them, the anime (or other media) would have to end. As a result, they are always protected by the plot, which means that they have plot armor.
However, that is not the plot armor that people are upset about. Plot armor actually occurs when a character should have died, but does not just because that character is needed for the plot. In other words, if it is not believable that a character could survive in a given situation, but they do survive for no good reason, then that character has plot armor. And it is not a good approach to writing stories.
Take, for example, Vampire in the Garden. One of the main characters, Momo, is constantly defying death and laws of physics because she cannot die, she has to live for the sake of the plot. But Momo is just a regular, squishy human; she is not Fine, who also gets her share of punishment, but who is also a vampire. Fine’s injuries can be waved away as some special vampire powers helping her heal. Momo, however, is constantly being shot at, falls from great heights, drowns in extremely cold water, and none of this seems to matter or even get explained in any manner. Are all the marksmen that tried to kill her incompetent? Is she especially good at holding her breath? Who knows, but this is clearly an example of plot armor done wrong.
On the other hand, if we take, for example, Hellsing, both 2001 and Ultimate versions, Alucard does not have plot armor. He is simply OP. In his defense, most characters he stands up against do not know how to kill a vampire, but he is shown to be basically unkillable even through complex blood magic shenanigans. To be fair, Hellsing did manage to put a leash on him, along with a seal that controls what he can do, but when the seal is released, he is basically indestructible.
To be clear, neither of those are necessarily a bad thing. Being OP may get a little tiresome at times, but with Alucard, for instance, they solved the issue by providing seals that determine what level of power he can execute, which allows for some variety in abilities. As for plot armor, at some point, you just exclaim, “no way could she survive this,” and it really hurts your investment in the anime. But a little plot armor does not have to break the immersion.