The source data shows decline in absolute numbers for pushes. 1432 last quarter, 19073 at peak Q3 2015. It seems to flatten out in 2018 and stays in the 1500-3000 range.
Most of the work moved the package ecosystem as the standard library shrank with many of the things in there moving to separate packages (for example, even the package manager is a separate package: https://github.com/JuliaLang/Pkg.jl). The result of this is that what you now see in Julialang/julia is mostly compiler work (and some tweaks to Base things like linear algebra overloads). With that said, it's quite an actively developed compiler!
Was JuliaCon 2020 remote? That probably has more to do with it. I can’t possibly imagine there are 40X more active users in one year. The niche it serves isn’t particularly large, either.
