Seems to be a former paid reward, as the parent post commentary indicates a bikini version available as a Fanbox reward, but here it is available on Twitter by the artist...
Rather posting "context" you could just explain the artist's reason you know. Especially it's written on something few will understand and require translation.
For short, the artist has a job and unable to update the fanbox has the artist is not allowed thus all paywall art are "free".