In 25w41a, you could remove `minecraft:knockback` using `set` and `negative values` under your requierements, but on the latest snapshot you can`t.

This is my code. It worked before the fix/”bug”

{
  "anvil_cost": 2,
  "description": {
    "translate": "enchantment.runes.familiarity"
  },
  "effects": {
    "minecraft:damage": [
      {
        "effect": {
          "type": "minecraft:set",
          "value": 0
        },
        "requirements": {
          "condition": "minecraft:any_of",
          "terms": [
            {
              "condition": "minecraft:entity_properties",
              "entity": "this",
              "predicate": {
                "nbt": "{Sitting:1b}"
              }
            },
            {
              "condition": "minecraft:entity_properties",
              "entity": "this",
              "predicate": {
                "nbt": "{Sitting:0b}"
              }
            }
          ]
        }
      }
    ],
    "minecraft:knockback": [
      {
        "effect": {
          "type": "minecraft:set",
          "value": -1000000
        },
        "requirements": {
          "condition": "minecraft:any_of",
          "terms": [
            {
              "condition": "minecraft:entity_properties",
              "entity": "this",
              "predicate": {
                "nbt": "{Sitting:1b}"
              }
            },
            {
              "condition": "minecraft:entity_properties",
              "entity": "this",
              "predicate": {
                "nbt": "{Sitting:0b}"
              }
            }
          ]
        }
      }
    ]
  },
  "max_cost": {
    "base": 27,
    "per_level_above_first": 0
  },
  "max_level": 1,
  "min_cost": {
    "base": 16,
    "per_level_above_first": 0
  },
  "slots": [
    "mainhand"
  ],
  "supported_items": "#minecraft:enchantable/weapon",
  "weight": 6
}

I won’t drop records because I can’t return to 25w41a.