When you next update your Discourse installation, the Glimmer Post Stream will now be enabled by default even for sites that still have incompatible customizations.
In fact, the entire widget rendering system will now be disabled, so any widget-based customization won’t be rendered anymore.
For now, on incompatible sites, the admin can re-enable the old behavior by modifying the values of the following settings:
glimmer_post_stream_modedeactivate_widgets_rendering
To enable the widget post stream again, both settings need to be changed.
This is the final phase before removing the old code from the Discourse code base, which is expected to happen in about a month. Subsequently, it won’t be possible to re-enable widgets anymore.