Not really, but there are ways around it: http://blog.quicklisp.org/2011/08/going-back-in-dist-time.ht...

I'm confused, is this about Quicklisp itself or about the packages it manages?

I.e. quicklisp 1.1 versus quicklisp 1.0 or libfoo 1.1 vs libfoo 1.0?

It's about the packages, a Quicklisp dist has a list of packages of specific versions (like libfoo 1.1, libbar 1.0, and libbaz 1.2) and you can switch to an older dist (which can have libfoo 1.0, no libbar, and libbaz 1.2).

Oh, so it's actually a carefully curated limited list? Because otherwise the combinatorial explosion would make this unmanageable super fast.