Magento2 - Class Magento\Catalog\Model\Product\Attribute\Backend\LayoutUpdate does not exist

The issue is in your database

I noticed in Magento 2.3.4, they created the LayoutUpdate file with the attributes custom_layout_update_file.

When we revert back, the attribute calling the file and cause the issue.

solution: Go to your Database and search the table eav_attribute

Run below query

SELECT * FROM `eav_attribute` WHERE `attribute_code` LIKE 'custom_layout_update_file'

You will find two records and just remove them (if you using Magento 2.3.2).