The bug

From the 19w13a snapshot changelog:

You can now get up to Bad Omen 5, and you only get 1 Bad Omen at a time from killing a captain. Each person that brings Bad Omen into the village will increase the Bad Omen level of the raid on that village up to Bad Omen 5. A higher level of bad omen means a higher chance that the enemies you face will have enchanted weapons.

However, it's possible to get Bad Omen level 6 in Survival mode by killing six illager captains. This appears to be unintentional since Bad Omen VI only seems to trigger a level 5 raid that will grant the "Hero of the Village V" effect to the player.

To reproduce

1. Kill 6 illager captains

   /summon minecraft:pillager ~ ~1 ~ {PatrolLeader: 1b, ArmorItems: [{}, {}, {}, {id: "minecraft:white_banner", Count: 1b, tag: {HideFlags: 32, BlockEntityTag: {Patterns: [{Pattern: "mr", Color: 9}, {Pattern: "bs", Color: 8}, {Pattern: "cs", Color: 7}, {Pattern: "bo", Color: 8}, {Pattern: "ms", Color: 15}, {Pattern: "hh", Color: 8}, {Pattern: "mc", Color: 8}, {Pattern: "bo", Color: 15}]}, display: {Name: '{"color":"gold","translate":"block.minecraft.ominous_banner"}'}}}], Attributes: [{Base: 0.0d, Name: "generic.maxHealth"}]}

   → Notice that you get Bad Omen VI

2. Trigger and defeat a raid
   → Notice that you get Hero of the Village V