The Plasmoid's Shape Self ability reads, in part:
As a bonus action, you can extrude a pseudopod that is up to 6 inches
wide and 10 feet long or reabsorb it into your body. As part of the
same bonus action, you can use this pseudopod to manipulate an object,
open or close a door or container, or pick up or set down a Tiny
object. The pseudopod contains no sensory organs and can’t attack,
activate magic items, or lift more than 10 pounds.
One thing that appears to be missing is any reference to casting spells with the range of touch. Barkskin, for example, affects a willing creature, so would be unlikely to qualify as a attack – neither is it activating a magic item or lifting any great weight. Bestow Curse allows for a saving throw, but requires no attack roll on the caster's part, so arguably that isn't an attack either. Shocking Grasp does require a melee spell attack, so that would likely not be an option for the Plasmoid.
Is there a blanket rule I'm missing that covers whether or not plasmoids could use their Shape Self ability to deliver touch spells, or would it be decided on a spell-by-spell basis?
Best Answer
Yes, it could
You've quoted all the relevant rules from Shape Self, and it doesn’t prohibit you from using the pseudopod to cast touch spells. But it’s more accurate to say you give yourself a reach of 10 feet, with some strict limitations, than that you extend the range of your spells.
The “allowed” actions are specifically things you get to do “as part of the same bonus action” used to extrude or reabsorb the pseudopod. Opening containers or picking up tiny objects etc are “free” things you can do at the same time as making or retracting the limb. But the pseudopod doesn’t automatically retract - indeed it has to exist for at least a full turn, since it’s a bonus action to extrude it or reabsorb it, not both.
Thus, while the pseudopod is extended, you should be able to use it for any action not prohibited by the Shape Self rules: so anything that’s not an attack or activating a magic item, and doesn’t involve lifting something that weighs more than 10 pounds.
That includes casting a spell, regardless of range, so long as it doesn’t involve an attack and you meet any other requirements of the spell. For example, since the pseudopod has no sensory organs, you will need to establish line of sight from your body’s position as normal.
As for what constitutes an attack, see the question What counts as an attack? In short, though, something is an attack if it involves an attack roll, or the rules describe it as an attack. So you are right that you could use the pseudopod to cast barkskin or bestow curse, but not shocking grasp.