Well, if 1.9 is technically not released yet, and so isn’t “fair game” to look for packages breaking, I looked at a few popular packages under Julia 1.8.
I took the General registry as 2021-06-01 (just ~1 year before 1.8) and tried to install and precompile — not even test — a few of the most popular Julia packages with Julia 1.8.
Lots didn’t work! Examples include CUDA, FiniteDiff, all of SciML, LoopVectorization, and even IntervalSets.
In my experience, this is the rule, not an exception — that lots of code (eg older package versions) transitively breaks when updating Julia. And this kind of breakage (doesn’t precompile and load) makes it impossible to do anything with those packages, not even a subset of their functionality.