Is there a base R function to dynamically order data.frame columns similar to dplyr everything()?

You can use setdiff:

df1[, c(keep1, setdiff(names(df1), keep1))]

df1[order(match(names(df1), keep1, nomatch = NCOL(df1) + 1))]
#  id month year v1 v2 v3
#1  1   jan 1999  A  2 10
#2  2   jan 2001  B  2 10
#3  3   mar 1984  C  2 10
#4  4   oct 1979  D  2 10
#5  5   dec 2019  E  2 10

Another option

df1[, unique(c(which(names(df1) %in% keep1), seq_along(df1)))]

or

df1[, unique(c("id", "month", "year", names(df1)))]