Is the migration build fully backwards compatible with a Vue 2.7 app when using compatConfig mode 2?

[vmoldoveanu]

Hi,
As the title says - is the migration build expected to provide fully backwards compatibility with a Vue 2.7 application when using compatConfig mode 2?
I'm getting some build errors related to functional templates and ":key" attribute.
Are those errors expected? I thought they would show after changing the compatConfig to "MODE: 3".
My vue.config is copied from the migration build docs:

  chainWebpack: (config) => {
    config.resolve.alias.set("vue", "@vue/compat");

    config.module
      .rule("vue")
      .use("vue-loader")
      .tap((options) => {
        return {
          ...options,
          compilerOptions: {
            compatConfig: {
              MODE: 2
            }
          }
        }
      })
})

Thanks!

[LinusBorg]

https://v3-migration.vuejs.org/migration-build.html#incompatible

At rhe bottom of that section's table it states:

COMPILER_SFC_FUNCTIONAL ⨂ is no longer supported in SFCs