Starting with Magento 2.3.0+, the use of a flat catalog is no longer a best practice and is not recommended

Question

Can anyone shed some more light on why Magento recommends against this now? Other than the short description they have in their dev docs.

Continued use of this feature is known to cause performance degradation and other indexing issues.

Flat indexers can cause:

• Heavy SQL load and site performance issues.
• Long running and stuck crons.

https://docs.magento.com/m2/ce/user_guide/catalog/catalog-flat.html

Answer 1

Check out the following tweets:

• The biggest problem here is not EAV or Flat tables, but the indexers. They fail very often and takes lot of time to dump data from EAV to flat tables.
  — Kalpesh Mehta (@kalpmehta) on August 31, 2019
• Quoting @VinaiKopp " The category page listings are built via the search engine in the open source version, too. That’s why the flat catalog table indexes are no longer needed. That has nothing to do with commerce vs. open source." He said this in another such tweet about this
  — Vikrant Shukla (@vikrantshukla) on September 1, 2019
• Main benefit was for filtering category product lists. This now is done through search engine. Flat table indexes no longer give performance improvements.
  — Vinai (@VinaiKopp) on August 31, 2019

And also the following post.

Hope this helps.