In a spoiler because it takes more space than I'd like it to, card images.
For poison:
:entropy Mutation is actually a double-edged sword, so it can be said to be balanced. No need to block it, much less with a shield that is doing something aside from protecting the player unlike the rest of the shields.
There is already a shield status called "Reflective", which reflects any direct spell damage to the player. Poison is not direct damage, but more of a status ailment. That's why Purify exists.
Also, mass anti-CC, we already have a card that isn't too harsh against domins but does its job well: