In #1213 updating mkdocs & dep versions a new GA/GTM setup needed to be used. Without a proper GA4 migration all the previous GA3 config was omitted under the impression it can't be gathering data anymore.

Fortunately Alphabet migrated GA3 UA properties to GA4 for those who never migrated it themselves. https://support.google.com/analytics/answer/11583528#zippy=%2Cwhat-happens-to-my-ua-connected-site-tag :

"For Google Analytics 4 properties that rely on Universal Analytics tagging for data collection, there is no additional risk when those Universal Analytics properties are shut down and deleted by Google. Once Google deletes your Universal Analytics property we’ll automatically continue to use your Universal Analytics tagging for your Google Analytics 4 property, however only if your UA tags are still on your website. If you decide to remove your UA tags, you can replace them with a Google tag."

There's even continued (limited) support for legacy analytics.js:

The template here overrides the defaults providing its own GA snippet based on the old analytics.js anyways, so this legacy path is still valid, just needs reconfiguring. (Expecting the GA3 being auto-migrated to GA4 and the analytics.js ingestion enabled by default as a result.)