From Magento 2.3.4, For the security enhancements, default custom layout update Textarea removed from the CMS page admin section.
If you want to add XML code snippet for the CMS Page, You must have to create a CMS Page layout handle for the page.
If you want to add Custom layout update code for the About Us page, You need to create a unique XML handle for the CMS page. Continue reading “How to add Cms Page Custom Layout Update file in Magento 2.3.4?”