"I cannot think of any mod or ability that would prevent these knockdowns"

-Inaros' augmented Scarab Armor

-Atlas just standing there

-Gauss' Kinetic Plating

-Nezha's Fire Walker (Subsumable, but only the lingering flames provide immunity = only useful when you stay in one area for some time)

-Nezha's Warding Halo

-Nidus' Parasitic Link

-Rhino's Iron Skin

-Frost's augmented 4

-Revenant's Mesmer Skin

-Oberon's Hallowed Ground

-Titania's Spellbind (Subsumable) (yes, it provides knockback/down immunity. Did you make sure to hold the cast?)

-Trinity's Link

-Yareli's Merulina

-Ember's augmented 3

-Nekros's augmented 4

-Ivara's augmented Quiver thing

-Hydroid's Tidal Impunity

-Valkyr's Hysteria (if the melee is out, so not very useful)

-Nyx's Assimilate

And for everyone else:

-Primed Sure Footed (Exilus)

-various other knockdown protection mods with less percentage)