variant_selection: {
          option1: "Black",
          option2: "XS",
          size: [
          "Black / XS", /// Only black color has XS size
          "Black / S",
          "Black / M",
          "White / S",
          "White / M",
          ],
        },

<span v-if="'variant_selection.option1 / {{ value }}' !== 'variant_selection.size.includes(variant_selection.option1) / {{ value }}' ">.</span>

<span v-if="variant_selection.size.includes(variant_selection.option1 + ' / ' + value)" ..

<span v-if="variant_selection.size.includes(`${variant_selection.option1} / ${value}`)" ..

const { createApp,ref } = Vue
const App = {
  setup() {
    const color = ref('')
    const colors = ['Black','White']
    const size = ref('')
    const sizes = ['XS','S', 'M']
    const selection =  {
          size: [
          "Black / XS", /// Only black color has XS size
          "Black / S",
          "Black / M",
          "White / S",
          "White / M",
          ]
    }
    return {
      color, colors, size, sizes, selection
    }
  }
}
const app = createApp(App)
app.mount('#app')

#app { line-height: 1.75; }
[v-cloak] { display: none; }

<div id="app" v-cloak>
  Color: <select v-model="color">
  <option v-for="c in colors">{{c}}</option>
  </select><br/>
  Size: <select v-model="size">
  <option v-for="s in sizes">{{s}}</option>
  </select><br/>
  Combination: {{ `${color} / ${size}` }}<br/>
  <span v-if="selection.size.includes(`${color} / ${size}` )">Combination is present</span>
</div>
<script src="https://unpkg.com/vue@3/dist/vue.global.prod.js"></script>