The most recent large breakage is that earlier versions of SortingAlgorithms.jl don’t work on Julia 1.9. SortingAlgorithms package has absolutely lots of dependents, and its earlier versions just don’t load on 1.9 at all.
This is exactly breaking existing user code, the same packages + versions that worked before now don’t.
For more examples, see Forward compatibility and stability of Julia vs. Packages - #42 by aplavin (and the whole tread, btw).