I've been playing since the first release, on and off. Things have changed quite a bit! Normally, I would find other time period NPCs by visiting wind shelter build locations, finding them wandering around. Then I'd use the strategic view to have an NPC rescue them, but that's not quite valid anymore.
The unlock screen simply says to find a chunk with them in it. I didn't find any new NPCs in missions, so I decided to check the neutral Ilari stones underground. I DID find other time-period NPCs down there! However, it didn't actually unlock the unlockable. They never moved, though they occasionally spoke, and I could glyph-transfer over to them, but it never counted as having met them.
I'm assuming I'm doing it the wrong way now. How does one go about unlocking NPCs from other time periods?
Edit note: This was during a multiplayer game.