Old versions of MacTeX are archived on the TUG server and a few other servers. To get an old version, use one of the addresses below. The http addresses tend to work better than the ftp addresses. When an ftp address leads to an error rather than a login dialog, it often means that your browser does not support ftp connections.
For more information, see the TUG page Historical Archive of TeX Material.
There are two ways to use these addresses. The first is to copy them to Safari and use standard web browsing techniques. Safari may put up a dialog asking for a login and password. If so, login as guest, and then no password is required.
The second method is to activate the Finder and select the last item in the Go menu, "Connect to Server." A login dialog will appear; click "Guest". An ordinary finder window will appear showing folders with various dates. Open the appropriate folder, and drag an install package to the desktop. For example, the 2015 folder contains three items, basictex-20150613.pkg, ghostscript916-20150607.pkg, and mactex-20150613.pkg. Packages are dated by release date in the format year/month/day.
After obtaining the desired package, you must disconnect from the server. The ftp server will be listed in the left panel of the Finder window. Select this server and click "disconnect".
Originally, MacTeX supported macOS 10.3 and 10.4, on PPC or Intel. Starting in 2011, MacTeX required 10.5 or higher, on PPC or Intel. The 2016 version of MacTeX is the last to support Leopard (10.5) and higher on PPC and Intel. MacTeX-2018 supported Yosemite, 10.10, and higher on Intel.
Early versions of MacTeX have a hidden folder structure, even though they appear to be single files in the Finder. These versions had to be compressed as zip files to be stored on the server. Later versions of MacTeX have a flat structure and thus appear as uncompressed single files on the server. Therefore when using ftp, recent install packages appear with a standard package icon, but older packages appear as .zip files that must be decompressed after downloading.
Happy TeXing on macOS !