
The possible OOTB way to remove New button on the home page is to control permissions for users in the site collection.
Go to Site permissions -> Advanced permissions settings -> Grant users with Read, Contribute or Edit permission.
After granting permissions for users, the New button will disappear on the home page.
As a reminder, controlling permissions for users in the site collection may affect custom Call-To-Action button on the page.
Due to the home page of SharePoint online site collection is in new experience, as a workaround, you need to deploy the SPFx web part to inject CSS/JS code to remove New button on the home page.
References:
https://github.com/pnp/sp-dev-fx-webparts/tree/main/samples/react-script-editor.
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