In what version of Oracle did spatial indexes become optional?
I can see that spatial indexes are optional in 18c.
Oracle 18c - Spatial Index No Longer Required
Creating and using a spatial index is no longer required for the use of any Oracle Spatial and Graph features. However, spatial indexes are highly recommended, and not using them can negatively affect performance in some cases.
However, you can require that a spatial index be used by specifying event 54700, as explained in Using Event 54700 to Require a Spatial Index for Spatial Queries.
But I'm wondering if it might have started in an earlier version.