Sorry went scavenging for food
Yeah there are plenty of maps that only open up when you are doing certain quests so there are some missable items but most things are easy to find, specially once you get that key item that reveals all hidden items, strange crystal or something it is called... There is no fixed order to all the different storylines though so you have plenty of freedom to focus on what you wanna focus on first.
most of the missable items are either common pointless items or items needed for other secret quests, or easter eggs and references other legendary games and pop culture stuff.
as a fail safe there is an ingame NewGame+ option so you won't have to start from scratch if you miss something.