How to enable visitors to see last published version during OOTB approval workflow if the Draft Item Security is set to Only Users Who can approve it. Currently no visitors are able to see the page until the approval workflow complete.
The scenario happens if you have no version 1.0 approved and published. If you have a version 1.0 approved and published, and a version 1.1 in approval workflow, users with at least read permission should be able to view the published 1.0 version.
If read access users get Access Denied browsing the published page, one could try deactivating and reactivating two Site Collection features: Workflows and Publishing Approval Workflow.