Hello,
Sorry but this is not entirely accurate, certain custom ISOs relating to versions prior to ESXi 6.5 may be downloadable from the websites of the respective system manufacturers, at the same time the customer holding a paid license may be able to download even earlier versions, I can, without any problem, download the ISO related vSphere version 5.1. Without prejudice to the fact that using them in practice requires an adequate license to be available.
Regards
Ferdinando