Will an already existing domain have a free Let's Encrypt certificate installed when extensions are configured as follows on the service plan level?
Keep websites secured with free SSL Certificate is enabled in Plesk > Service Plans > example_plan > Additional Services:
Option secures both newly created domains and existing ones by execution of the scheduled task configured in Plesk > Tools & Settings > Scheduled Tasks:
/opt/psa/admin/bin/php -dauto_prepend_file=sdk.php '/opt/psa/admin/plib/modules/sslit/scripts/keep-secured.php'
It runs every hour and checks if a domain has an SSL certificate. Hence, if certificate installation fails, it will try to install it in one hour.