What's the purpose of the new legacy option introduced in @intlify/unplugin-vue-i18n v1.6?

[kingyue737]

https://github.com/intlify/bundle-tools/tree/main/packages/unplugin-vue-i18n#legacy

@intlify/unplugin-vue-i18n already supports vue 2 before. I don't understand why introducing another legacy in config. What's the difference between legacy and bridge?