These rules are currently in configs for Vue 2 (in all of essential, strongly-recommended and recommended), but not in configs for Vue 3:

• vue/no-custom-modifiers-on-v-model
  → could be useful as an uncategorized rule for Vue 3 to disallow custom modifiers, so let's not deprecate it
• vue/no-multiple-template-root
  → could be useful as an uncategorized rule for Vue 3 to prevent attribute inheritance problems, so let's not deprecate it
• vue/no-v-for-template-key
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: key Attribute)
• vue/no-v-model-argument
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: v-model)
• vue/valid-model-definition
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: v-model)
• vue/valid-v-bind-sync
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: v-model)

I didn't find any uncategorized rules that would only be useful for Vue 2.

Thus, deprecating them is not technically a breaking change, but I think it will receive more attention if we do it in a major version.