Amusingly, download.info.apple.com still appears to serve all the files that use...

Amusingly, download.info.apple.com still appears to serve all the files that used to be on the old FTP site, including a small and somewhat random subset of old system software versions for Mac and Apple II, Newton firmware updates, etc., but directory browsing isn't enabled, so you need direct links, which you can get from the Wayback machine[1].

But once you have the links, you can download the actual files directly from Apple, e.g., if you're ready to upgrade your Mac Plus to the latest and greatest version of System 6, download

https://download.info.apple.com/Apple_Support_Area/Apple_Sof...

[1] https://web.archive.org/web/20141025043714/http://www.info.a...