Roger explained the situation, during the discussion on github, here:
https://github.com/stm32duino/Arduino_C ... -522423866
It seems the GDPR makes handing it to someone else legally impossible, among other reasons.
The main legal issue is with people's personal details, stored in their user accounts, rather than with the readable content. That's also available, partly, via the "Wayback Machine", at archive.org, so reproducing the content has already been done, and presumably does not infringe copyright.
However, we do have the read-only archive of the old site, thanks to PeteS (aka Loud160, in that thread). It's incomplete, but there's still a lot of it. That's now here:
If you click a link, and find yourself back at that page, it's because the correct page is missing.
So, for background reading, that's a good place to start - but bear in mind that a lot of it is talking about the Maple / Maple Mini core, or Roger's derivation of it, after Leaflabs closed down. There are some differences between his core and the shiny new official ST core, though you'll probably work out fairly quickly what no longer applies.