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I've been thinking of an idea for a campaign where the adventuring party must find pieces of a weapon: a sword that would be used to slay a lich, as it has the power to destroy a persons soul.
When it is completely forged, it reveals itself to be a Githyanki silver sword that they then have to use within a certain amount of time, or face the wrath of an entire race of angry Psychic space people.
Can such Githyanki silver sword be used to kill a lich, or other similar "immortal" being?
Githyanki silver swords are known for being able to cut off a creature's astral cord. This allows their wielders to kill a foe which is currently in astral form, such as through the Astral Projection spell. A lich's condition has nothing to do with astral form, so the sword won't do much with its special ability.
But that's just plain boring, so...
As the DM (or right now, the creator of the campaign), you are entirely free to create new items, and especially so for story reasons. What you're describing right now is one of those kinds of special objects that make the plot move forward, which we often call a MacGuffin.
It does not matter whether or not any such silver sword could kill a lich, because this is The Shattered Silver Sword Of Plot, which is a unique weapon capable of killing a lich. Or you could rule that liches in this campaign interact in such a way with those silver swords, or even that this weakness is specific to this singular lich. Go wild.
Functionally this comes down to a DM's ruling or interpretation.
A literal or mechanical reading of the rule interaction would be something like:
Only if the target is on the astral plane or has traversed through the astral plane to another realm, so that a cord is present. Only if the entity has a body present on the material plane - And then only if they have a soul - or if separating the soul from the body would "Kill" the creature.
Arguably a lich - by definition of their phylactery - could survive having their soul separated from their body - but what happens to such a soul is unspecified.
But as described in Astral projection - the severing of the cord only separates the connection between the soul and the body, which results in an outright kill. Would an immortal deity or other such being have such a weakness? So again - it comes down to DM's interpretation.
For side-by-side reference:
Stats are in Mordenkainen's Tome of Foes:
MACIC ITEM: GREATER SILVER SWORD
Weapon (greatsword), legendary (requires attunement by a creature that has psionic ability)
This magic weapon grants a +3 bonus to attack and damage rolls made with it. While you hold the sword, you have advantage on Intelligence, Wisdom, and Charisma saving throws , you are immune to being charmed, and you have resistance to psychic damage. In addition, if you score a critical hit with it against a creature's astral body, you can cut the silvery cord that tethers the target to its material body, instead of dealing damage.
*note that the effect only severs the cord - it does not state an instant death effect.
Astral Projection:
Your astral body resembles your mortal form in almost every way, replicating your game Statistics and possessions. The principal difference is the addition of a silvery cord that extends from between your shoulder blades and trails behind you, fading to Invisibility after 1 foot. This cord is your tether to your material body. As long as the tether remains intact, you can find your way home. If the cord is cut-something that can happen only when an Effect specifically states that it does-your soul and body are separated, killing you instantly.
Note: It is uncertain if the "death" effect of having your cord cut is a property of the cord itself - or a property of the astral projection spell.
