Discourse is now shipping with a more modern implementation of the header, including more flexible and robust customization APIs. The new version has already been automatically enabled for sites with compatible themes & plugins.
For incompatible sites, a banner is being shown to admins warning them to take action before the old header implementation is removed. If you see this banner, follow the linked instructions to update your customizations. If you don’t see the banner, then your site is ready, and you don’t need to do anything.
Warning banners have been shown to admins since 20th June (v3.3.0.beta3).
From the week commencing 5th August (v3.4.0.beta1), we will enable the new header by default for all sites, even if they have incompatible customizations. It will still be possible for admins to switch back to the old header by manually setting the “glimmer header mode” site setting to “disabled”.
From the week commencing 2nd September, the option to use the old header will be removed entirely.
If you need help getting your site ready, please post in the instructions topic or speak to your hosting provider.