That specific PR is part of Thomas's push to speed up ggplot, specifically at the gtable level, but also more generally. plyr is likely not going anywhere for some time because the last I saw there were a few functions that have not found a new home in the new tidyverse (dplyr/tidyr/widyr). I don't remember which one, but I want to say adply. I would say that outside of big fixes that plyr hasn't seen a lot of love for some time, but i've really not been paying attention to it.
Is there a specific function you are concerned about?
HTH,
B