To duplicate the functionality in plyr, you will likely need to look in (at least) dplyr in addition to purrr. There's isn't necessarily a one-to-one relationship between functions, however. In a broad sense, plyr focuses on the shape of your input versus the shape of your output, while the tidyverse (and dplyr in particular) often works best with data frames/tibbles as both the input and output.
That means that the exact functions needed to replace ddply depends on what you are trying to do with it. The one commonality is that you would typically use dplyr::group_by to first group the data. After that, you might use dplyr::summarize to create a new data frame with each unique combination of your grouped columns along with one or more new columns calculated with one row per group. Alternatively, you might use dplyr::mutate to add a column that uses a per-group calculation, such as dividing one column by the per-group minimum from that column.