The documentation tells me: If you would like to use RottenLinks with MediaWiki 1.39, you have to use the REL1_39 branch.
The link goes to https://github.com/miraheze/RottenLinks/tree/REL1_39
Then, the result is:
PHP Fatal error: Uncaught ExtensionDependencyError: RottenLinks is not compatible with the current MediaWiki core (version 1.39.6), it requires: >= 1.40.0. in /var/www/test/mediawiki/includes/registration/ExtensionRegistry.php:432 Stack trace: #0 /var/www/test/mediawiki/includes/registration/ExtensionRegistry.php(276): ExtensionRegistry->readFromQueue() #1 /var/www/test/mediawiki/includes/Setup.php(278): ExtensionRegistry->loadFromQueue() #2 /var/www/test/mediawiki/maintenance/doMaintenance.php(83): require_once('/var/www/test/m...') #3 /var/www/test/mediawiki/maintenance/rebuildLocalisationCache.php(286): require_once('/var/www/test/m...') #4 {main} thrown in /var/www/test/mediawiki/includes/registration/ExtensionRegistry.php on line 432
Now what?