This explains it, Optim is holding back StatsBase.
⌅ [2913bbd2] StatsBase v0.32.2 (<v0.33.21): Optim
Either remove it or solve why Optim 0.20.1 can’t be updated to latest 1.7.3.
Side note: you should work with projects: you seem to have too many packages in the default project, causing compatibility issues like this one.
In the mean time, I think we should drop StatsBase 0.32 compat in Plots.
EDIT: dropped in this PR: drop `StatsBase` 0.32 by t-bltg · Pull Request #4423 · JuliaPlots/Plots.jl · GitHub.