My interpretation of AU is a story when the characters have decided to take a different path than the original timeline. Especially when it comes to "What If's", there's always something more than meets the eye.
For example, Quirkless AU Izuku fell down into a different realm and his goal is to go home.
Or Giyuu decided to quit the demon slayer corps.
To me, as a normal shipper and a story lover. I'm so glad to be more normal than these.... Whatever they are. I always focus on details when it comes to films, series, AUs, or stories. I also love slow-burns, totally real to me.