I asked a very similar question, regarding Unobtainable Items in general, to Delany when we interviewed him. You can see his answer here:
Sometimes, it is a long-term plan of the developers to introduce items they want to use later to expand a story. Other times, circumstances might have changed that made it necessary to alter a previous plan and “ready-to-use” items are a remnant of that. In few cases it also may be a simple mistake.
—Delany Interviewed by the English TibiaWiki
At the time, I had items such as the Guidebook in my mind which probably fall on the last case Delany mentioned ("a simple mistake"). Regarding the Blue Sphere, I believe the devs changed their plans for this item and it ended up being unobtainable.
Concerning how and when the sphere became obtainable, I remember it very well. In 2019, when CipSoft was reevaluating the list of contest prizes with fansite admins, I suggested adding the sphere to the list. It seems the content team liked my proposal since they allowed fansites to award the blue sphere from January 2020 onwards.