Magento ceased the use of a flat catalog from Magento 2.3.0 onwards and does not consider it as the best practice now. It was analyzed that continuous use of a flat catalog causes degradation and other indexing issues. Earlier, the flat catalog was primary to be used for improving performance as it simplifies the required data for products and categories. Now, since Elastic Search is being used, the flat catalog has become problematic, leading to indexers failing, taking too long to index and deteriorating the performance. The given blog tells how the flat catalog moved out of the scene from Magento 2.3 onwards.
Disclaimer: The above article has been aggregated by a computer program and summarised by an eCommerce specialist. You can read the original article at Lexiconn