We already have the immutable repo that you are looking for. We do all our builds from a caching proxy server that pulls in new binary objects when we need them

We already have the immutable repo that you are looking for. We do all our builds from a caching proxy server that pulls in new binary objects when we need them, but serves up the old one for the 99% majority of repeat builds. Artifactory is one package that does all of this, but in the past I have rolled my own using various packages to cache maven repos, Ubuntu packages, Python Pypi packages. It is not that hard and makes you mostly unaffected by Internet outages and repos that go away for some reason. And since the cache is under your control you can publish your own binary objects to it, and clean it up too if you really want to get rid of something.