None of the collectable stuff is permanently out of reach which is actually pretty important to know from the start. The game has these special backtracking points scattered throughout the main areas, and they're always accessible even after you've pushed forward with the story content. What's interesting is how the collectables are spread across different zones - you'll notice certain patterns in their placement once you start actively hunting them down.

Keep in mind that thorough exploration really pays off here since most hidden items have subtle environmental hints pointing to their location. The devs clearly designed the progression so that players wouldn't need to stress about missing anything crucial, which is a nice touch especially for completionists. When you're ready to go back for anything you missed, just use those special backtrack points and make sure to check those slightly-off-path areas, since that's usually where the good stuff is hidden.

You might want to mark spots on your map where you notice something suspicious but can't quite reach it yet - chances are you'll get the right ability later to access those areas.