The terminology isn't as black and white as it sounds, it's complicated. But the bit about shadows is new, so it's clearly not a perfect reflection of the concept. In my own world, under highly specific conditions, a mage can summon a hero of the past to fight as their contracted familiar of sorts. They share an empathic link, with the Master serving as the magical power source for the Servant to exist in the living world. If a Master dies, so too will their Servant, but the reverse doesn't automatically apply.

Heroic Spirits have existed since the beginning of human history; for as long as there have been legends and heroes. But it's not as if every single mage can summon one; only seven Servants can exist in the world at a single time. [laughs in apocrypha, extra, and fgo] So it isn't a matter of realms connecting, or of such figures being automatically connected to people.