I think it depends more on the content of the story than on the year. For instance, a story set in the 80s about a family drama could easily slide into the modern category with no issue, assuming 80s technology and culture doesn't feature heavily. A story set in the 80s about an alien invasion will probably be historical, since 80s technology and culture will by nature feature heavily.
A question to ask yourself is "could I set this story in the modern day without making major changes to the plot?" If the answer is no, then your story relies on historical elements and is historical fiction. If yes, it may still be historical fiction depending on how heavily you use the aesthetics of your time period.
Target audience is probably most important. Are you targeting people who like historical stories or people who like modern stories? Someone who wants an English political epic about the aftermath of the battle of hastings won't click on your historical story about the stock market crash of 2008. Even though that's technically a historical story, you'll probably get more people interested in it if you put it in the modern category.
