For example, could you today run a ruby based SSG from 10 years ago? Let's pretend there is a bit of custom code and time invested in it.

I guess you could install a bunch of old versions on Ruby, gems, etc. on your machine too, but docker, or a VM might be easier.

Or you could go through, remember how all the code works and upgrade it to the latest versions of everything to run it. Assuming the SSG du jour 2013 is still maintained :-).

I tried to resurrect a 2 year old NodeJS project and gave up trying to upgrade it to the latest versions, NodeJS people really love their major version changes, and breaking back compat for no real reason.