This package was moved to be a sublibrary in OrdinaryDiffEq.jl. The migration is a maintanance change but should have no functional change to the library.
DiffEqBase.jl is a component package in the DiffEq ecosystem. It holds the common types and utility functions which are shared by other solver packages to promote code reuse in the differential equation solver code. Users interested in using this functionality in full should check out DifferentialEquations.jl.
The documentation for the interfaces here can be found in DiffEqDocs.jl and DiffEqDevDocs.jl.