From Magento 2.3.4 onwards, there is no more the layout Update XML field in the admin panel. you will find the Custom Layout Update dropdown on the CMS Page Edit, Category Edit, and Product Edit pages. It was done to increase the security level of the system.

So now if you want to add layout Update XML then You need to create an XML file in your theme or module level and assign an XML file to the Custom Layout Update selector.
How to add the Custom Layout Update file in Magento 2.3.4?

1) For product pages syntax:
E.g catalog_product_view_selectable_10-TEMP_CUSTOM.xml

2) For category pages syntax:
E.g catalog_category_view_selectable_10_CUSTOM.xml

3) For CMS Page syntax:
E.g cms_page_view_selectable_custom-page_CustomPage.xml