There are 12 Days Gone Nero Checkpoint Locations for World's End Trophy. This guide shows all of them and their solutions to access them.

They contain Nero Injectors (gives an upgrade to health, stamina, focus) and become available as fast travel points.

You must always find a fuel canister and use it on the power generator to open the doors (sometimes you must also find a fuse, just follow the clues from the fuse box).

None of these are missable! After the story there's free-roam and you can go back to all regions and finish up any side activities and collectibles you have left. It's worth noting that there are two temporary points of no return. One at the end of 3rd region "Lost Lake" and one more at the end of 5th region "Crater Lake". This will lock you out of some regions until the end of the story. Then after the story the whole map opens up again.