How enable eventcart extension in WordPress CiviCRM?
As of this writing event cart should be enabled but hidden, and currently still relies on the setting under Administer - CiviEvent - Component Settings to switch on and off features. You can verify if the extension is enabled by running this SQL: select is_active from civicrm_extension where name='eventcart';
If it's not, in civi go to Support - Developer - api explorer v3 and choose Entity=Extension, and Action=Enable, and for the key parameter put eventcart. Then click execute at the bottom. The upgrade should have done that though. There should be a record that it did that in ConfigAndLog.