According to Google’s spokesperson, in a statement to The Register, the Basic HTML version of Gmail is essentially the older version of Gmail that is still in use. It lacks some features and has been replaced by the current version of Gmail for over a decade.
TLDR: Google will discontinue its Basic HTML version of Gmail in January 2024, redirecting users to the Standard view. The Basic HTML version was designed for users with slow internet connections or loading issues. It will be replaced by the current version of Gmail, which offers more features.