Hi, could you clarify where/when you mean by "back then" (particularly since you mention "fantasy")? Equipment has varied greatly across time and geography , so zeroing in on a specific war or culture during a given time period (e.g. Ancient Greece? the Norman Conquest? etc) would be of great help to anyone attempting to answer here. Thanks!

This Reply by myself covers the effectiveness of plate armour in medieval Europe. Put simply plate armour, and mail, and thick cloth armour (the sort that would be used as a primary defense) was proof against sword slashes. Thrusts from a sword were dangerous not so much because they could penetrate the armour but because thrusts could pierce through the joins and other gaps in armour if they were well-aimed.

This Reply by u/MI13 covers some techniques used to get around plate armour by swordsmen.

As always I am happy to answer follow-up questions.