I don’t think it is that impressive, though I have the most experience with Java, where a fat jar on a random, obsolete website displaying some university material will just work.
To make your comparison with fat jars closer to the point, imagine you got the source code from the owner of that old obsolete website, and need to use it in your newest Java project, with the bleeding version of Java, gradle and all the stuff.
The Go 1 promise[0] says that the language spec and set of core libraries will be backwards compatible. I maintain a few dozen go projects, some of which are 10 years old, and while upgrading from i.e. 1.4 -> 1.18 has been relatively easy, it definitely has not been zero work. There are changes in the tooling, dependency ecosystem, deprecated dependencies that should be replaced, runtime behavior differences, things like that. But the language itself and stdlib is totally stable.
It just worked.
And code was quite readable. I mean it not as a complement to myself, rather the fact that Go is designed to be readable and it pays off.
I only had to add modules support (basically run `go mod init` in the library's folder) to make it 100% modern-version friendly.
That was as close as I could be to successfully complete 10 Years Reproducibility Challenge [1].
[1] https://github.com/ReScience/ten-years