Let me put this in context for you about those legendary chests. You'll mainly want to watch for specific timing during atreus-only sections since that's where things can get tricky. When you're playing through these sequences, remember those chests you see there technically can be missed during that first run, but here's the key detail - any important items like runic attacks or relics will show up at the dwarven shops afterwards so you're never truly locked out of the good stuff.

The really important timing comes down to two specific areas you'll want to keep track of - that first angrboda sequence in ironwood and odin's chamber in asgard. Once you clear those zones they're permanently sealed off so make sure to grab what you need while you're there. For everything else though you've got plenty of chances to circle back through the realms, and the game actually helps you track what's truly missable with special markers for things like ravens and berserker gravestones. Maybe interesting to note that for that sweet 100% completion you only need about 30 out of 35 nornir chests and just the legendary ones with yggdrasil jewels for fixing up your amulet. The rest are nice to have but won't hold you back from getting everything done.