Google recently introduces Analytics v4.

ga4 no longer provides IDs compatible with analytics.js which are in the format UA-#####-#

ga4 now uses a new gtag.js and IDs in the format G-ABC123

Google no longer allows creation of properties with the older analytics IDs. Existing Google Analytics properties can be upgraded and converted to ga4 which will replace their UA-#####-# ID with a G-ABC123 ID. There is no way to downgrade.

Google is emailing existing users encouraging them to upgrade to ga4. To my knowledge at this time they have not announced any kind of cut off date for when migrating to ga4 must be completed.

Currently it is possible to use template overrides to put the new ga4 into an mkdocs site, however the

google_analytics:
  - UA-#####
  - auto

format is extremely convenient and easy to use (by comparison to the slightly more advanced setup of overrides).

For the purpose of making the transition possible without breaking existing setups without warning, I think it would be great to add

gtag:
  - G-ABC123

options to mkdocs.yml that would use the new gtag.js which is documented by google to be implemented as

<script async src="https://www.googletagmanager.com/gtag/js?id=G-ABC123"></script>
<script>
    window.dataLayer = window.dataLayer || [];
    function gtag() {
        dataLayer.push(arguments);
    }
    gtag('js', new Date());
    gtag('config', 'G-ABC123');
</script>

This would allow existing mkdocs users to continue using google_analytics: for as long as google continues to support it, or upgrade to gtag: very easily at their own discretion.

Alternatively it would also probably be file to implement something in google_analytics: handling that uses gtag depending on the ID specified.