Blizzard have detailed upcoming changes to Paladin and Warrior Holy Shield and Spell Block talents.
Paladin and Warrior (Source)
An upcoming build is going to have changes to how the Holy Shield talent and “Spell Block” works. The Holy Shield talent is not changing, but how Holy Shield interacts with magic damage is changing.
Holy Shield is themed as being Spell Block, however in Shadowlands, it isn’t actually a real blocked attack under the hood. This has been changed to work similar to how standard physical blocking works, and there are some changes as a result of this:
You will actually have to have a shield equipped to benefit from Holy Shield (This is very minor, but noting for posterity)
You can no longer Spell Block attacks from behind, while stunned, or while casting/channeling a spell.
Holy Shield will no longer allow you to block periodic spell damage.
Holy Shield will now allow you to block creatures that deal non-physical auto attack damage, such as if a Fire Elemental melee attacks you as Fire damage.
Holy Shield will work against attacks that deal combinations of physical + (some other element) of damage, such as a Fire+Physical spell or attack.
The combat log will properly report Spell Blocks from Holy Shield as blocked damage, rather than absorbed damage.
Holy Shield will continue to use your character’s actual block chance, and your character’s armor value for purposes of how much block value you have.
These changes will also apply to the new Protection Warrior Spell Block talent.