Keep the page that was the first touchpoint the cookied lead had with the site

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Thanks Adam. It was the last: {{trigger.web page}}

You can do this already right now. You create a visited webpage is any smart campaign, then in the flow steps you add a change data value & a trigger token. I'm not sure on the specific token, you should test it out, whether it is {{trigger.trigger}} {{trigger.name}} or {{trigger.web page}} - but probably the last. Then set that campaign to run flow only 1 time & set the field to block updates. Then you have exactly what you're searching.