According to CTFTime 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 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 and had a search. So I got the GitHub Repo of this site. I inspected commits of main branch, and found that in commits named minor typo and minor typo 2, 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}