Isabelle had one more secret on her youtube account, but it was embarrassing.
Finishing previous OSINT Chals will assist you with this challenge
The first two characters of the internal of this flag are 'hi', it may not be plaintext
The flag capitalization may be different, please be aware
The description is a big hint on what we should do. Using the word "had" implies that the flag cannot be found on the Youtube Channel presently. Thus, we can use the Web Archive to search for things in the past.
Searching on the Web Archive, we notice that there was a snapshot of her Youtube channel on the 14th of July. Looks promising.
Looking at the website link, we notice that it is different from the one we found on Isabelle's Bad Opsec 2 challenge.