The snippet now looks much closer to gtag.js. What changed in the code:
gtm.js loads through a script tag written directly into the HTML, so the browser can start downloading it earlier.
The noscript iframe after the opening body tag is no longer required.
The start event now uses gtag('js', new Date()) instead of pushing gtm.start to the dataLayer.
gtag() is defined inside the snippet, so the Consent Mode default can be set without extra code.
The dataLayer name is no longer a parameter. Renamed dataLayers need &l=name added to the URL and the variable changed by hand.
Watch out: if your CMP defines its own gtag(), you may get duplicates. With a strict-dynamic Content Security Policy, the static script needs a nonce or an allowlisted domain. Setups that customized the old function will need a rewrite.
My tip: before replacing the snippet in production, check in Preview mode that your Page View triggers still fire.
This comes from a screenshot analysis, not an official Google announcement.
Spotted by Matteo Zambon.
