In my theme the StatusMessages element is rendered without the class message--status.
Looking for the source of the problem I find in status-messages.html.twig this line
@see template_preprocess_status_messages()
which is exactly where I would like to look
however that function doesn't exist in d8.
So saying, I can't see any clues in \Drupal\Core\Render\Element\StatusMessages either
Fixed
11.0 🔥
system.module
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The classes were removed intentionally in
#2407743: Remove classes from system templates s*.html.twig →
. When this issue was opened it was still common to use classy as the base theme, so this may have seemed like a bug, but now we have starterkit_theme which includes the 'messages--' ~ type class.