I will cut this short. Always get permission. The moment you write a Fan Fiction, you are opening yourself up to a ton of problems.
While on the traditional line, it is generally fine on the popular end. On the lower and unofficial side, that is another situation.
If you do not get permission, then you are opening yourself up for potential problems.
Note, in both sides. You’re taking a risk. It is that on the official and popular side, the authors tend to be fine with it. However, that isn’t a guarantee.
Can’t say in the unofficial side.
Depending where, the laws differ and there might be exceptions under certain constraints.
Anyway, point is. Even if it was abandoned, try and get permission. The other way is wait an X amount of years after their death.