According to [CTFTime](https://ctftime.org/) or Google, we know that the official site of this contest is
https://sdc.tf/
First of all, according to the challenge description, I want to use [Wayback Machine](https://archive.org/web/) to inspect this site. There are 4 or 5 snapshots of this site, but I just used Ctrl+F search to find flags but got nothing.
And then, I suspected whether the flag is in TXT record of DNS, and use `dig txt https://sdc.tf/`, but got nothing.
Finally, I had a sleep. When I woke up, I suspected whether this site might use [GitHub](https://github.com/) and had a search. So I got the [GitHub Repo](https://github.com/acmucsd/sdc.tf) of this site. I inspected commits of main branch, and found that in commits named [minor typo](https://github.com/acmucsd/sdc.tf/commit/3744178fdf4e360c1e9972b7e476b7fed27a5ec6) and [minor typo 2](https://github.com/acmucsd/sdc.tf/commit/cb86546ced8e238c88b7af51191788abda8dc461), there were file changes like adding and deleting flags. When I used the flag to submit, it was correct.
So the flag is:
```sdctf{Th3_L0$t_trE@$ur3_0f_th3_INt3rnEt}```