If you are looking for a singular historical fact, I think the most common one (at least among ‘ancient’ scholars) was the historicity of Troy from The Iliad, which most 19th and even early 20th century historians regarded as purely mythological. (The description of the Trojan War from is generally thought to be an amalgamation of conflicts from different eras, and ‘Homer’ himself a ‘historian’ who brought together different oral traditions into a single epic poem centered around a romantic conflict rather than a series of resource conflicts over centuries and even different versions of Troy.)
In the broader sense, it is difficult to highlight “posited certain truths” that were universally accepted because historians love to disagree. The view of ‘colonization versus genocide’ is certainly a perspective that has changed but that is in large measure because the social perspective of the cultural superiority of white Europeans is no longer considered the default, and the evidence of the cultural and technological innovation of other cultures has always been there even if dismissed and even literally plowed over. For Western historians, I think the biggest overall shift in perspective has been the characterization of the European ‘Dark Ages’ following the collapse of the ‘Western’ Roman Empire, when in reality there was a flourishing of cultures due to innovation and migration in Central and Northern Europe, as well as the rise of Islamic cultures in the Levant, Arabia, North Africa, and around the Mediterranean from the 8th through 16th or 17th centuries, not to mention everything going on in East Asia.
The recognition of large empires with elaborate trade networks, great stone and earthwork constructions, and advanced astronomical, mathematical, and agricultural knowledge and innovations is newer because of the degree to which those have been suppressed, both culturally and by literally destroying much of the evidence of the cultures of the Americas, but I think the historical and archeological community today is in broad agreement that there were great empires with expansive trade networks and elaborate irrigation systems mostly hobbled by a lack of draft animals and ready access to iron and tin, limiting them to copper and stone as structural and working tool materials.
Stranger