There are a lot more than 45 footnotes in this game, by the time I finished the game I had about 55 in my Lore tab, so in theory you could miss a few side quests or NPCs and still get it.
Unfortunately, you actually need to hit Y
on each footnote as it comes up during conversation for it to actually count towards the 45 total. If you do not press Y to open the footnote, you will NOT get credit towards the achievement.
Even though the footnote then shows up in your Lore tab.
So make sure you are pressing Y to open and close the footnotes during conversations or else you will get screwed over like me