Earlier you could download any magento version from Download page in Tech Resources section. But not so long ago Magento has removed the official links and reworked download page. Now you can download only 2.x-related resources like source code / sample data / patches. This is part of Magento Software Lifecycle Policy.
If you are still using Magento 1 and looking for a certain release, please check github repo from OpenMage – https://github.com/OpenMage/magento-mirror/releases. It contain releases starting from 1.1.1 up to 184.108.40.206.
Security patches can be found in Magento patches mirror repository – https://github.com/akai-z/magento1-open-source-patches
More useful Magento 1 repositories
Awesome Magento List – this list has everything you might ever need for magento. Great links compilation starting from patches, security and testing and up to blogs / books.
Docker image for Magento 1.x
modman (Module Manager)
MailChimp For Magento 1