The attribute minecraft:attack_knockback
doesn't seem to be working in a consistent way when applying knockback to attacks blocked by a shield. If a mob has any value of attack knockback above 0, the knockback applied to a player blocking isn't proportional to the attribute value. Normal hits work as expected (but for some mobs don't, see MC-269226).
This is not consistent with ravagers and hoglins/zoglins, which have an attack knockback base value above 0 and deal knockback to players blocking with a shield.
How to reproduce
Summon a husk with an attack_knockback of 5, switch to survival.
/execute summon minecraft:husk run attribute @s minecraft:attack_knockback base set 5
✔ The husk attacks you applying high knockback to you.
❌ If you are blocking with a shield, the husk doesn't apply any knockback.
Expected result
Mobs with an attack knockback value above 0 would knockback players blocking with a shield, like ravagers and hoglins/zoglins.
Observed result
Mobs with attack knockback don't deal increased knockback to players blocking, despite normal hits doing so.
Linked issues
Comments

Relates to MC-147694

Isn't the shield meant to block attacks? If so, why is this unresolved? It makes sense for the shield to block knockback. I assume this is going to have a resolution of "Works as Intended"

It's meant to block attacks, yes, but this attribute overrides the knockback resistance of the shield, so in that way ravagers, hoglins/zoglins and wardens can deal you knockback while blocking. This behavior is also a mechanic and not a bug, judging by the triage of MC-217567.
I can confirm this. Also affects 21w43a.