Pagers are now created using a the pager.manager service, as also reported in The pager has now a backend changeable service → . The old API has been removed from Drupal 9; modules which use it need to be updated for Drupal 9 .
The page.manager service lacks of tutorials. It would be useful if the Pager Example module could illustrate how to use this service.
Active
4.0
Pager Example
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It's likely this issue predates Contrib.social: some issue and comment data are missing.