Demiplatonic can be an orientation on its own or can combined with other orientations. For example, one could be demiplatonic and homoplatonic, meaning that when one does experience platonic attraction it's only towards people of similar genders.
It is unknown who first coined demiplatonic, however it has been used among the a-spec community since at least 2017.
The flag was coined by an unknown through pride-color-schemes tumblr account on April 16th, 2017.
The prefix demi- means half or partial. The prefix is also used in words such as demigod.