As a free user and the owner of a systematic review or literature review, when you delete a Reviewer, their decisions disappear from the collaborative systematic review. However, inviting them again under a different role (as a Collaborator or a Viewer) would show up these decisions again.
If you can't or don't want to add the Reviewer back to the review, you can export the review and check the Notes field in the articles file or the customizations_log file, where all the decisions will be listed. Please note: the blind should be on in order for Reviewers' decisions to be exported.
Another option would be to subscribe to a Premium Membership, where we offer you the ability to revoke a Reviewer's access without removing their decisions from the review.