It's always hard to prove the absence of something during specific period but it's pretty unlikely platinum was ever widespread or even know during the medieval period.

Platinum is significantly scarcer than gold elementally. It often occurs in small deposits within or alongside gold ore deposits and was historically discarded as a byproduct or impurity in gold mines in Europe, Africa, and Asian. Today a majority of newly mined platinum is still extracted as a byproduct of larger gold mining operations.

Some pre-Columbian artifacts have been found in Ecuador made of a white gold-platinum alloy; these smiths may have understood it is a sperate metal, but that is not entirely clear. The use of platinum in this area is likely due to higher occurrences of large mostly pure nuggets. The Spanish "discovered" the element after finding nuggets in Peru in the 1700's while mining for gold.

I'm not aware of any pre mid to late 1700's old World artifacts made of pure(ish) platinum.

Working platinum is radically different from gold or silver due to Platinum having a melting point of 3,215°F compared to gold at 1,948°F. It's a significantly more difficult metal to smelt, mold, and work. Only the top craftsmen and workshops would have likely been able to work with it even if they were away of it and it's properties.