What is this I don't even...
The Loyalists don't need a shield-specific equivalent upgrade just because the Rebels have extra shield regeneration.
The Rebels are the defensive Advent faction, almost all of their tech is aimed towards survival: better shield regen, repair stations can repair shields, destroyed ships can be brought back to life, planets get income and culture boost when you defend them, etc..
The Loyalists are the OFFENSIVE Advent faction, for them to have some shield mitigation upgrade just because the Rebels have some nice shield upgrades would not make sense. You know what they do have? They've got a culture rate almost twice as strong as anyone else because they want to forcefully convert the entire galaxy into the Unity, they've got weapon boosts because killing things is what they're supposed to do.