Yuri Deity is a tag about a mortal being, like us, having powers that draws godlike yuri related artworks. To the individual fans, they are respected by their fans as if they are a godlike individual, hence the origin of this tag.
Similar to Breast God or Ass God, but think the yuri version of it.
Basically any artworks that portray a scene of a girl interacting with another girl that deserves universal praise will be tagged with this tag.
Alternatively, it also refers to Namori