Classic forms are forms that use the classic user interface of SharePoint, as opposed to the new Modern user interface. Classic form features can only apply to classic forms. If a modern form is configured for the list, it will be used in place of the classic form and no classic form features will be active. Modern forms offer a significantly wider set of capabilities, with the exception of View Permissions.
Classic forms are not currently being deprecated and can be used for existing and new projects. However, newer features will be added to the modern forms only, so consider converting your forms to Modern, when convenient.
NOTE: in Microsoft 365 version, permissions and rules cannot be based on Azure AD groups, only individual users and SharePoint groups are supported.
