At the very bottom of the download page, there is link (very small font text)
* Download
......
Old Versions
It will link you to page like this
https://www.reaper.fm/download-old.php?ver=4x
where e.g.
4x in the address is the number of major version (so 7x, 6x, 5x, ... etc.).
Also at the top of the that page there is a title
"REAPER 7.x downloads" on the left and
"Jump to: latest 7.x 6.x 5.x 4.x 3.x 2.x 1.x 0.x" on the right. Those numbers are links to specific versions.