How to prevent scales::percent from adding decimal

Perhaps not a direct answer to your question, but I have used scales::percent_format and its accuracy argument ("Number to round to") in similar settings.

mtcars %>% 
    count(cyl) %>% 
    mutate(prop = n / sum(n)) %>% 
    ggplot(aes(x = cyl, y = prop)) + 
    geom_point() + 
    scale_y_continuous(labels = scales::percent_format(accuracy = 5L))

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I think the behaviour of percent was changed in scales 1.0.0. See NEWS and updates in code here.

Just an update, scales::label_percent(accuracy = 1L) will round to the whole number, scales::label_percent(accuracy = 0.1L) will round to the first decimal and so on.