If Grapple can prevent Plane shift, who needs to be grappled, and where on their body?
As I understand the rules, denying a caster the use of BOTH arms would DEFINITELY prevent casting plane shift or any other spell with a somatic component (also, the caster can't grab a material component they aren't already holding). Can a grapple prevent a caster from using one or more arms?
If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures.
Casting some spells requires particular objects...
A spellcaster must have a hand free to access these components, but it can be the same hand that he or she uses to perform somatic components. (PHB p.203)
The spell Plane Shift has some additional requirements, though:
You and up to eight willing creatures who link hands in a circle are transported to a different plane of existence. (PHB p. 266)
Those are some specific targeting requirements! I can think of three additional ways that the Grappled status might cause problems: Zeroed movement, mis-identified targets, and precluded hand-holding.
As I understand the rules, the fact that a grappled creature "can't move" does NOT prevent transport to another plane. See Does the Blink spell break grapples?
The targeting requirements of plane shift looks for grasping hands. The Grappled status involves the grappler's hand grasping the grappled target. Will plane shift attempt to target a creature that grapples either the caster or one of the creatures in the hand-holding circle?
The Grapple rules don't dictate what body part the grappler grabs. If the plane shift caster or one of the spell's intended targets has one of its arms grappled, though, it seems likely that it will not be able to comply with the requirement that all plane shift targets be holding hands. By the RAW, can/will a plane shift fail if the caster or one of the circle targets has either of its arms/hands grappled?