The problem is, to be specific, that the Plots “pyplot” backend is not just a front-end to the PyPlot package - it mostly bypasses PyPlot and calls direcly into matplotlib via PyCall (and LaTeXStrings), which usually was not a problem, because those were loaded as dependencies of PyPlot.
On 1.0, though, (Pkg3 to be exact) you can’t import a package that was installed as a dependency - you can only import packages you’ve explicitly installed. That changes the installation procedure - we’ve updated the error message on the newest release of Plots (https://github.com/JuliaPlots/Plots.jl/pull/1656/files) but it may not be shown appropriately.
So it’s not Requires-related.