Yes, high kicks are the most recent addition and mainly were included for show and sport. My first MA was the "old" TKD, the type that Gen. Hong Hi Choi extracted from Funikoshi's interpretation of karate. Early TKD was essentiallly Shotokan karate-do. Kicks were never higher than the waist, and generally targeted the hip bone (to control and turn the opponent), knees, ankles and insteps (stomps). There was a lot of powerful punching.
As the Japanese occupation ended, Koreans were quick to separate themselves from Japanese influence and started to alter their TKD-karate into something more visibly Korean. That included de-emphasis of hand techniques (and for many dojangs, the loss of the sources of upper-body power was lost as well), and splicing the retained low kicks with some of the remnants (so the legend goes) of the old Korean taekyon kicking sport-art used as a royal court entertainment in the feudal period.
They also changed the way the former Japanese kicks (which were Okinawan kicks, which were Chinese kicks
) were chambered, to allow for a more mobile, on-the-move style of fighting. The high and spinning-high kicks (wheel kicks, jump spinning axe kicks...) were added here as well, mostly as a crowd pleaser for demonstrations.
By the late 1970s, TKD had divested itself entirely of karate. But it also sportified itself and many schools and systems of TKD had become more sport than MA.
I wrote earlier that very few people can use high and spinning kicks effectively against a good jujutsu guy/grappler, MMA fighter and basically any good MAist who has developed a keen eye for the timing and tactics of kickers. Kickers are among the (if not THE) most vulnerable of fighters, because picking one foot off the ground and commiting to its use places the fighter on only one leg.
It's risky, and not worth cultivating oneself as a one-trick pony in hopes that you can use those high kicks as your main source of fighting. Better to be very well versed in other things and save the high kick to finish off your opponent with a flourish when he is already dazed...