可以使用mapply + :创建序列：

library(dplyr)
library(tidyr)
df %>% 
  mutate(enrollment = ifelse(deregister != 9999, mapply(`:`, enrollment, deregister), enrollment)) %>% 
  unnest_longer(enrollment) %>% 
  mutate(deregister = replace(deregister, enrollment != deregister, 9999))

#   id enrollment deregister
# 1  1       2014       9999
# 2  1       2015       9999
# 3  1       2016       2016
# 4  2       2011       9999
# 5  2       2012       9999
# 6  2       2013       9999

这里是一个替代方法。请检查一下这个方法是否适用于您的原始数据集：

library(tidyr)
library(dplyr)

  df %>%
    mutate(value = ifelse(deregister != 9999,
                          enrollment + 1,
                          0)) %>% 
    pivot_longer(-id, names_to = "name", values_to = "enrollment") %>% 
    group_by(id) %>% 
    arrange(enrollment, .by_group = TRUE) %>% 
    mutate(deregister = ifelse(name=="deregister" & enrollment!=9999, enrollment, 9999)) %>% 
    filter(enrollment != 9999 & enrollment != 0) %>% 
    select(-name) %>% 
    ungroup()

id enrollment deregister
  <dbl>      <dbl>      <dbl>
1     1       2014       9999
2     1       2015       9999
3     1       2016       2016
4     2       2011       9999
5     2       2012       9999
6     2       2013       9999