As soon as a collaborator is deleted, their decisions disappear from the collaborative systematic review. However, inviting them again (as a collaborator or a viewer) would show these decisions again.
If you can't or don't want to add the collaborator 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. Note that the blind should be off in order for collaborators decisions to be exported. Another option would be to subscribe to a premium membership, where we offer the ability to revoke a collaborator's access without removing their decisions from the review.