Are there any areas that become sealed off during the course of the game?

Solution 1:

There are two places you cannot go back and explore. The starting junk world(Nothing you can miss) and the last place you travel to for the finale, that place has some scanable objects that are specific to that area. Its linear so very hard to miss them.

Solution 2:

You can go back to every area of the game except the very last area and the very first area. Both of these areas do not have any collectibles so you do not have to worry about missing any The last mission takes place in a new location (there are no collectibles, echoes or otherwise here so you don't have to worry about missing anything). After you beat the final mission it puts you back to the point in the story just before you do that mission, though you can't play it again for whatever reason. So the dialogue reverts back to what it was before the final mission as if it hasn't happened yet. There a re good tutorials online for getting back to any location you don;t know how to get back to. Just search the name of the location and there will most likely be a YouTube video that can bring you there or walk you through the surrounding areas.