When you sacrifice it to Skirk Prospector, it returns with a -1/-1 counter, but this cancels out against the +1/+1 counter provided by Metallic Mimic or First Day of Class
Ive been very critical of Azorius Control decks early into the format, as the lack of other win conditions besides Teferi, Hero of Dominaria is concerning
Cast it with extra mana to make it huge, and you can remove counters to draw cards and create robot tokens and fuel both your board presence and card advantage
Sheoldred can also allow its controller to draw their entire deck, without losing due to self-mill, when combined with Lichs Mastery
The Commander Rules Committee has stated that this is because it is very hard to kill more than a single player with poison, especially because your opponents likely are not piling on with their own infect creatures
Its generally better to buy individual booster boxes over cases