1) Looking at the code in Drupal\aggregator\Entity\Item, several fields have called setDisplayConfigurable('view', TRUE). However there is no "Manage Display" page in the GUI to configure these fields - the annotation has no entry for field_ui_base_route.
Note that "Manage Form Display" and "Manage Fields" would not make sense for aggregator item - only "Manage Display" is wanted.
2) The default configuration is missing for core.entity_view_display.aggregator_feed.aggregator_item.default.
Active
2.0
Code
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.