Anyone getting the argument of `across()` is deprecated error may like to try...

X %>% mutate(across(all_of(lead_cols), \(x) str_pad(x, width = 3, pad = "0")))

or more simply in this case (noting the overworked variable naming)

X %>% mutate(across(x:y, \(x) str_pad(x, width = 3, pad = "0")))