This hierarchy contains various items of interest in TeX history, from individual files to entire systems. See the TUGboat article by Ulrik Vieth for an overview.
The master is at (FR):
and it is mirrored at (US):
Using a mirror is recommended. The tug.org server has strict limits on the number of ftp connections, so if you have trouble connecting (e.g., to get an old TeX Live or MacTeX), please try one of the mirrors. Be aware that browsers generally report some vague problem on the server when the actual problem is simply too many connections.
More mirrors would be most welcome; cron
rsync -a --delete rsync://tug.org/historic/
and email webmaster at tug.org when it's set up. Thanks.
MiKTeX releases later than April 2017 (2.9.6300) are available on github.
Also, the yearly proTeXt release on the TeX Collection DVD includes a complete MiKTeX installation, through the 2019 DVD. These are in the directory historic/systems/protext here.
By the way, an (entirely independent) repository of material from SAIL covering 1977-1990 is available. Here are some of the directories there relating to TeX.
System directories on SAIL:
Personal directories of Don Knuth:
This trivial README (index) file is public domain.