The issue where the gtag.js is not detected in Shopify stores is simply due to Shopify’s integration using a legacy code.
Meaning Shopify is not using gtag.js.
To resolve this issue, you can install Google Tag Manager for Shopify, including the steps to install Google Analytics version 4.
What Happens If You Ignore This?
Nothing serious will happen as it is only a warning from the perspective of Google Ads. Google does this to encourage website owners to use new coding instead of relying on old coding.
Your Google Analytics Universal property will still work as usual when you do or do not upgrade the tag.
Should You Install It?
Suppose you want to stop receiving warnings in Google Ads, complaining that there is no gtag.js or GTM tag on your website. Yes, you should, as there is no harm in installing the code.