The Sheikah Heirloom is an item from The Legend of Zelda: Breath of the Wild. It is piece of ancient Sheikah technology originally known as an Ancient Sphere that has been passed down within Impa's family for generations. According to Paya, an ancient prophecy states the Sheikah Heirloom will grant a gift of antiquity to a hero chosen by the heirloom. Impa's family has served as its protectors during eight generations. Impa herself is quite protective of the heirloom as she requests Link to respect the rule not touch it despite his esteemed and heroic status.
At one point, Paya who had apparently figured out its function attempted to use the Heirloom to activate an ancient dias platform near Cotera's Great Fairy Fountain but her attempt failed though it is unclear if it was before Link's activation of the Great Plateau Tower caused all Sheikah Towers to raise restoring power to the shrines or her not being the prophecized hero that contributed to this failure. After Link appeared, Paya began to suspect Link might be the prophecized hero as she knew he had been previously chosen by the Master Sword thus a prime candidate for fulfilling the prophecy.
The Sheikah Heirloom plays a prominent role during the Shrine quest "The Stolen Heirloom". One of Impa's bodyguards, Dorian, has stolen the Sheikah Heirloom under the orders of a Yiga Blademaster. He was a former member of the Yiga Clan who killed his wife and forced him to spy on Kakariko Village by threatening kill his two daughters Koko and Cottla after Dorian betrayed the Yiga Clan by marrying a member of the Sheikah that remained loyal to the Royal Family of Hyrule. He eventually refused to continue spying for them, so they blackmailed him into stealing the heirloom by threatening the lives of Paya and Impa. Link defeats the Yiga Blademaster and uses the Heirloom to activate the Lakna Rokee Shrine near Cotera's Great Fairy Fountain, fulfilling the prophecy freeing Impa and Paya's family from their role as the Heirloom's protectors.
As the Sheikah Heirloom is known to have been passed down within and protected by Purah, Impa, and Paya's family for generations Purah and Impa had presumably protected the Heirloom before the Great Calamity. Its status as a piece of ancient Sheikah technology may have even played a role in sparking Purah's interest in Sheikah technology and ultimately her career as a Sheikah technology researcher. However Purah's impulsive habit of impulsively charging ahead especially when inspired may have caused the Heirloom to be passed down to her more patient and responsible younger sister despite Purah being the firstborn. Alternatively their family may have been prohibited from removing the Heirloom from Kakariko Village due to its intended purpose thus Purah may have forced to leave the Heirloom with Impa who remained in Kakariko Village while Purah went to live in the Hateno Ancient Tech Lab. Additionally, Purah may have completed her studies of the Heirloom long before the Age of Burning Fields which presumably may have been the first piece of ancient technology she studied given it protection by her family. Purah presumably may have figured out its intended purpose though likely recognized it was designed to be used by its chosen hero. Also Impa is very protective of the Heirloom and is known to have kept her impulsive elder sister from causing trouble with her experiments thus Impa might not have allowed Purah to test her theories concerning the Sphere's function or failed like Paya did as Purah admits she herself along with Princess Zelda failed to understand the Shrines of Trials required power supplied by the Sheikah Towers which were buried until Link activated the Great Plateau Tower which confounded research efforts and the Heirloom like the Sheikah Slate and Shrines may have been designed so only the hero could wield them for its intended purpose as implied by the prophecy tied to the Heirloom.
It is unclear if Impa suspected Link to be the prophecized hero but given her patient wait and see approach to life as noted by Purah, presumably Impa suspected that Link was the prophecized hero fated to use it than it would occur in its own time putting her faith in the prophecy while patiently guarding the Heirloom until the prophecy was realized. Additionally by not allowing Link to use the Heirloom, its theft occurred allowing Link to discover Dorian was being threatened by his ex-clansmen allowing Link to intervene and save Dorian while also eliminating a Yiga Blademaster saving Dorian and the Heirloom before fulfilling the prophecy. Thus in being patient Impa allowed Link to help Dorian deal with his dark past and protecting Kakariko Village from the threat posed by the Yiga Clan while also fulfilling the prophecy.