I am using Yarn 1.21, and with a couple of manual steps I am able to make the offline mode work in a completely offline environment (corporate network).
Yarn 2 looks very exciting but after some googling I cannot find any mention of this.
Also looking through the Yarn 2 docs I can't see any mention of an offline mode.
Does Yarn 2 support an --offline mode?!?
I found out that Yarn 2 will support an offline mode, which is referred to as the Yarn cache, from this answer below thanks to arcanis:
the offline mirror has been merged with the Yarn cache,
so any time you see the word "cache" you can think of it
as what was previously called the offline mirror
https://github.com/yarnpkg/yarn/issues/7872#issuecomment-582385100