I am an English major. For this reason, using correct grammar is something that I really do pay attention to. Now, with that said, am I someone who is going to be all in your face about active versus passive voice? Absolutely not. However, grammar is something that, frankly, many authors are not paying attention to, and may be the reason for their rejection letters.
Grammar checkers are now becoming something more people are paying attention to, especially with the introduction of AI programs that proclaim their ability to write amazing documents.
I started thinking about it this weekend as my son was working on his Master's Thesis. His professor, sent the partial rough draft back to him claiming it was full of a massive amount of grammar mistakes. But here is the issue. The professor was using one of those "grammar checkers" out there that many universities are using. The "mistakes" that were supposedly there were not mist, but general suggestions that really focused on a personal approach to writing. The other issues stemmed from "potential mistakes" in complex and compound sentences.
The problem here is that grammar checkers, of any sort, are looking for "trends" in the writing. These look for theoretical patterns in the writing that "may or may not be an issue." In fact, if you use a grammar checker, the message that pops up does indeed suggest that it "may be a issue" but does not say that it is definitely an issue.
I think it is also important to remember that unless an author has personally made adjustments in his or her grammar checker/spell checker, it is not going to find all of the mistakes. A good example of this would be finding fragments in writing. MS Word does not traditionally look for fragments because, in many pieces of business writing, fragments are justifiable. Even if you did look for fragments in a piece of fiction, we have to remember that we speak and talk using fragments.
So, what does it come down to? You do need to know grammar. BUT, it still requires you to have some common sense to the writing. You can often trust your own gut instincts.