两件事：
1.大部分你想做的事情都可以作为一个向量来做，不需要申请;

1. ifelse是类不安全的，例如，尝试将它与Date-class（或POSIXt-class）一起使用，将剥离类并返回数字。参见How to prevent ifelse() from turning Date objects into numeric objects。
   我建议这是一个替代方案：

out <- rep(as.Date(NA), length(t1))
out[grepl("/", t1)] <- as.Date(t1[grepl("/", t1)], format = "%d/%m/%Y")
out[is.na(out)] <- as.Date(as.numeric(t1[is.na(out)]), origin = "1900-01-01")
out
#  [1] "2020-02-14" "2020-02-17" "2020-02-18" "2020-02-19" "2020-02-20" "2020-02-21" "2020-02-26" "2020-02-27"
#  [9] "2020-02-28" "2020-02-05" "2020-03-05" "2020-04-05" "2020-05-05" "2020-06-05" "2020-09-05"

如果您有更多的候选格式，您可以考虑https://stackoverflow.com/a/52319606/3358272和https://stackoverflow.com/a/70304571/3358272，它会迭代可能的格式（以类似的方式），并尝试转换所有格式，直到完成（或耗尽）。
使用base::ifelse（去除类）的替代方案是使用dplyr::if_else或data.table::fifelse，如果您将任何一个包用于其他用途，这可能会更简单。请注意，它们将在所有t1上运行这两个方法，因此您将得到警告（两个实现）。

if_else(grepl("/", t1), lubridate::dmy(t1), as.Date(as.numeric(t1), origin = "1900-01-01"))
# WARN [2023-05-17 09:54:01] {"msg":"uncaught warning","warning":" 6 failed to parse.","where":["ccbr()","if_else(grepl(\"/\", t1), lubridat","lubridate::dmy(t1)"],"pid":"39316"}
# WARN [2023-05-17 09:54:01] {"msg":"uncaught warning","warning":"NAs introduced by coercion","where":["ccbr()","if_else(grepl(\"/\", t1), lubridat","as.Date(as.numeric(t1), origin ="],"pid":"39316"}
#  [1] "2020-02-14" "2020-02-17" "2020-02-18" "2020-02-19" "2020-02-20" "2020-02-21" "2020-02-26" "2020-02-27"
#  [9] "2020-02-28" "2020-02-05" "2020-03-05" "2020-04-05" "2020-05-05" "2020-06-05" "2020-09-05"
data.table::fifelse(grepl("/", t1), lubridate::dmy(t1), as.Date(as.numeric(t1), origin = "1900-01-01"))
# WARN [2023-05-17 09:54:11] {"msg":"uncaught warning","warning":" 6 failed to parse.","where":["ccbr()","data.table::fifelse(grepl(\"/\", t","lubridate::dmy(t1)"],"pid":"39316"}
# WARN [2023-05-17 09:54:11] {"msg":"uncaught warning","warning":"NAs introduced by coercion","where":["ccbr()","data.table::fifelse(grepl(\"/\", t","as.Date(as.numeric(t1), origin ="],"pid":"39316"}
#  [1] "2020-02-14" "2020-02-17" "2020-02-18" "2020-02-19" "2020-02-20" "2020-02-21" "2020-02-26" "2020-02-27"
#  [9] "2020-02-28" "2020-02-05" "2020-03-05" "2020-04-05" "2020-05-05" "2020-06-05" "2020-09-05"

这可以通过用suppressWarnings包裹整个if_else/fifelse来抑制。