I contacted support and they advised that at this stage they have no update when their system will accept the new Google Analytics 4 (GA4) measurement ID.
It is disappointing because clients who don't have a business or eCommerce plan don't have any option to add the GA4 tag in.
Support suggest clients add a UA tag in but this disregards the fact GA4 is based on a global tag and even Google recommends during setup not to use the UA tag.
Understand Google has made the change, but at the very least Squarespace should allow code injection until they have a fix for this as GA4 is now the default for all new Google Analytics users. They are also strongly encouraging every current GA user to apply a GA4 property in parallel.
Another alternate, would be if Squarespace allowed people to add Google Tag Manager in without the need to upgrade their plan.
Given the current situation, is not suitable for several clients to pay more just for this feature.