I have recently create a solution to integrate a legacy angular 10 project with angular 20 using angular element.
Github Link: https://github.com/jimkeecn/legacy-angular-migration-web-component

Since there are two angular, so I remove zone.js from the angular 20 project and in my angular 10 project I always check the polyfill and zonejs file if were loaded. if loaded, the polyfill will not be needed for angular 20.

Here is the question, I know in this senario, we are forcing the angular 20 web component using the old out-dated version of polyfill and zonejs. That means some new API patch may not works. Will it be an big issue? Will it outweight the benefit of having ability to slowly migrate legacy system part by part instead of update at once? any security concern?

In my project I have verify some library update and to check if signal works in the legacy. As the zoneless feature will be in production ready in version 21, we kind of remove the biggest headache for zone.js conflict issue. Does it mean, from now on, we can use this kind of approach to migrate old/big angular project slowing?