I am trying to find how one of my historic pages has customized the left navigation on Confluence?
You cannot customize the custom navigation of spaces in the page. That's why you cannot find it in the page history.
All navigation are configured in the space and general administration areas.
Confluence does not have any revisions of this.
Our add-on Menu Navigation Editor has it. You can find the information to that in this link: https://infos.seibert-media.net/display/Productivity/Menu+Editor
And you can find it also at the Atlassian Marketplace: https://marketplace.atlassian.com/plugins/net.seibertmedia.confluence.plugin.menuEditor/server/overview