composition apiでは、ナビゲーションガードの記述も少し変わっているようです。

今までの書き方

  beforeRouteLeave(to, from, next) {

  }

CompositionAPI

■ 公式ドキュメント
https://router.vuejs.org/guide/advanced/composition-api.html#navigation-guards

import { onBeforeRouteLeave, onBeforeRouteUpdate } from 'vue-router'
import { ref } from 'vue'

export default {
  setup() {
    // same as beforeRouteLeave option with no access to `this`
    onBeforeRouteLeave((to, from) => {
      const answer = window.confirm(
        'Do you really want to leave? you have unsaved changes!'
      )
      // cancel the navigation and stay on the same page
      if (!answer) return false
    })

    const userData = ref()

    // same as beforeRouteUpdate option with no access to `this`
    onBeforeRouteUpdate(async (to, from) => {
      // only fetch the user if the id changed as maybe only the query or the hash changed
      if (to.params.id !== from.params.id) {
        userData.value = await fetchUser(to.params.id)
      }
    })
  },
}

若干書き方が変わったり、インポートしたりする点が違いますね。