Some data from the site analytics (available to users with 25,000 reputation) regarding close reasons:

In the last 90 days (that's as far as it goes back), the distribution of close reasons has been:

Of the site-specific close reasons (i.e. those beginning with "Not suitable for this site") the one for not being about science fiction or fantasy is by far the most common.

In the list of 17 custom close reasons that were applied during this period, only two of them were about Future Works. By contrast, six of them were some form of "it's not science fiction or fantasy".

Of course, there may be some confounding factors here, such as if users are using the "Opinion-based" close reason to cover Future Works.

While it is not strictly necessary, since you can just use a custom close reason, I see no downside to having an additional close reason for the generic "this is not about science fiction or fantasy".

Some data from the tools available to users with 10,000 reputation regarding close reasons:

In the last 90 days (that's as far as it goes back), the distribution of close reasons has been:

Of the site-specific close reasons (i.e. those beginning with "Not suitable for this site") the one for not being about science fiction or fantasy is by far the most common.

In the list of 17 custom close reasons that were applied during this period, only two of them were about Future Works. By contrast, six of them were some form of "it's not science fiction or fantasy".

Of course, there may be some confounding factors here, such as if users are using the "Opinion-based" close reason to cover Future Works.

While it is not strictly necessary, since you can just use a custom close reason, I see no downside to having an additional close reason for the generic "this is not about science fiction or fantasy".