The etymology of the very casual greeting 「ヤッホー」 appears disputed and undecided. Various theories and hypotheses have been suggested. This site lists German, Hebrew, and 山伏 as possible sources. A lot of places claim it was a mountain climbing term (this, this, and many others). My question is when did it become popularized among young people as a fashionable greeting? When did become a thing in Japan? Where did it start? Did it start among members of a subculture (e.g. various teen subcultures in 渋谷/原宿)? I know the term has been around for at least 10, 15 years.
No one says yahoo in Japan. Some decades (around 100-70) ago , Yahoo was used for making echo or voice call someone in the distance at mountain site. Yahoo is recognized as a calling in laud voice for distance , so It may use for joking. Never used as a popular greeting. Decent people does not yahoo for greeting.
The word itself has been widely recognized for a long time as "something you shout in the mountains". Even novels written in the 1950's have examples of ヤッホー.
In town, it may be used very occasionally as a humorous, unique greeting. Well, sometimes people feel おはよう is too uninteresting and want to say something different. I may have heard ヤッホー used in this way once or twice in the last 20 years. However I don't think it has ever been "popularized among young people as a fashionable greeting" in the 21st century. This is an old word everyone knows since childhood, and as far as I know, there is no reason for it to suddenly become a fashion. Theoretically, there may be a community (e.g., fans of a singer) where ヤッホー is used as a popular greeting, but I am not aware of such an example.