For 2.3 and earlier, the link is also to the Sphinx documentation, but there are links from the Sphinx index to the C++ API documentation.
The most user-friendly solution to this would probably be a landing page with the same structure as the current version documentation index, like we do for the development version documentation (https://cantera.org/documentation/dev-docs.html).
The link to documentation for Cantera 2.4 from the main documentation page (https://cantera.org/documentation/index.html) directly links to the Sphinx documentation only, and doesn't provide a way to get to the Doxygen documentation for 2.4, even though this is present (https://cantera.org/documentation/docs-2.4/doxygen/html/).
For 2.3 and earlier, the link is also to the Sphinx documentation, but there are links from the Sphinx index to the C++ API documentation.
The most user-friendly solution to this would probably be a landing page with the same structure as the current version documentation index, like we do for the development version documentation (https://cantera.org/documentation/dev-docs.html).