en.wikipedia.org/wiki/Krvna_osveta

The claim says the practice started in the Balkans only in the 15th century under Ottoman rule. The available legal-historical evidence contradicts that.

• A legal-history source notes that Nemanjić-era legal monuments from the 12th century already contain the institution of composition for blood crimes called vražda. That places the practice, or at minimum its legal handling, in the region centuries before the 15th century.
• Another source on Dušan's Code states that in medieval times, revenge in blood was a common way of punishing murder, and that vražda functioned as compensation that ended the blood feud. Dušan's Code is from the 14th century, again predating 15th-century Ottoman rule.
• The Serbian government's history page also places the Ottoman Turks as a later "new threat" arising only after medieval Serbia had already reached its apex in the mid-14th century. So a practice documented in medieval Serbian law cannot have originated only after Ottoman rule began.

So the sentence is too late by several centuries. A more defensible formulation would be that Ottoman rule may have affected, revived, or prolonged blood-feud practices in some areas — not that the practice itself started then.