It's unclear when Google decided to discontinue Basic HTML support, but information about it can be found on this support page titled "Use the latest version of Gmail in your browser".

This time the scythe is coming for one of the company’s oldest features—Gmail’s basic HTML view. In January 2024, that simple interface heads for the great beyond.

Google updated a Gmail support article to say that the basic HTML view for Gmail will be discontinued in January 2024.

Before you begin building your page, you should brush up on some basic HTML rules. Here's some helpful information to get you started. The HTML markup language provides a backbone for web ...