DESCRIPTION files don't actually solve this problem - if you're trying to peg a package version, and it inevitably ends up an outdated version in the future, you'll be greeted with that error "Package 'xxx' is not available". Why? That version has been archived. I'm sure you've run into this error at some point.

devtools I guess is a solution (write some code to iterate over package requirements), but let me ask you this - how do you protect against breaking changes in devtools?

I have yet to encounter something more reliable than MRAN snapshots https://mran.microsoft.com/documents/rro/reproducibility