I've already completed the game's main story and am mopping up the last collectables and achievements. The link above states that the history note is in the dam's control room which was a location you needed to access to complete "The Long Drop" treasure hunt. I've already done that mission and didn't find the note then because I wasn't looking for it at the time. To enter the control room, "The Long Drop" required you to defeat a locked door by shooting an interior padlock from an exterior window. However, it appears that once you complete that mission, the door becomes sealed and there is no longer a padlock to destroy. Thus, there is no entrance to the control room that I can find so it appears that gaining all collectables is now impossible.
If true, I'm extremely frustrated to get this far and find out that 100% completion is no longer achievable. Ubisoft did this in Far Cry 3 where a collectable was hidden in the cave where the escape boat was kept. If you didn't pick it up before completing the main story, it became impossible access the cave and collect afterwards. Ubisoft took a lot of grief for that design decision and patched the game to give you credit for it even though you never picked it up. Ubisoft seemed to have learned their lesson for a time and never repeated this blunder in subsequent games. That is until FC 6.
If my understanding of this situation is correct, Ubisoft needs to fix this now.