OK well this seems to be gnarly but I found a workaround elsewhere. Long story short, I had to add Pkg.add(name="OpenSSL_jll", version="3.0") to my CI.yml. Explanation below.
This seems to be a super recent bug, or maybe an old bug in new clothes, and I’m not sure anyone knows exactly what’s causing it. If people are inclined to “pass the buck”, the following packages may be involved: julia-docdeploy, Documenter.jl, OpenSSL_jll.jl, Git.jl, as well as possibly upstream OpenSSL packages, etc…
The bug has been reported by github user tamasgal to Documenter.jl but closed as a duplicate of a 2-year-old bug that is still open there.
Github user tamasgal made a minimum working example which now includes their workaround. The workaround is to force Julia to install an older version of OpenSSL_jll.jl before deploying the documentation to github.
I’m leaving this discussion here because the workaround is virtually unfindable now, since it is buried in a discussion of a closed issue on github.