The query filters and any other Linq operations that translate expressions to SQL cannot include custom functions or properties since SQL cannot execute that code. So you either need to write your filters in a way that can be directly translated, like Svyatoslav's comment, or if you want to associate them to an entity or something like a base Entity class:

public abstract class SoftDeleteEntity
{
     public int Id { get; protected set; }
     public DateTime? DeleteTime { get; set; } = null;

     public static Expresssion<Func<TEntity, bool>> IsDeletedExpr => (e) => e.DeletedTime != null;
}

Assuming you have a base class for your entities that support soft deleting, otherwise this can be put in the entity(ies) directly.

Then when you want to apply the query filter or a Where clause:

modelBuilder.Entity<MyModelName>()
    .HasQueryFilter(SoftDeleteEntity.IsDeletedExpr);

And that should do the trick if you want the filter more encapsulated.