I solved it by using the x-forwarded-proto header in the load balancer.

This is tested by the is-https package:
https://www.npmjs.com/package/is-https

Are there plans to keep supporting this in the future?

From my point of view, using x-forwarded-proto is enough, so it doesn't seem necessary to force https.

@Timkor why not just set the "hostname" parameter?

sitemap: {
    hostname: 'https://example.com',
    // ...
}

@NicoPennec
Ok, then I will keep it that way.

I have multiple domains connected to my nuxt app. Like:

example.com
example.de
example.fr
example.it
example.es

@Timkor I closed the initial topic. But you have an interesting use case and so I need more details for a future feature 😉

Do you need multiple sitemaps or always the same sitemap for each domain name (except for a dynamic hostname) ?