There is a kick I enjoy employing, that I call a vertical front-kick, though it may have other names. I do have full splits currently, as I am easing back into determined, hardcore training.
But I have found despite losing certain aspects of my flexibility, so long as I continued repeating my kicks, I would not lose the flexibility in terms of kicking, which is the point of those stretches.
I suspect the reason for my ability to kick at that height is because of the butterfly stretch. For about the last decade I have been able to keep my legs rested comfortable with heals at my groin. I also augment the stretch by sitting atop my feet for a time.
And stretch which will allow you to further the flexibility of how far you can reach your knee to your body, the higher you will find your ability to kick. I think in part this is why TKD practioners focus on the stretch so much, and I honestly think it works.
I stretch once every morning for about 30 minutes, going through every stretch I know for about 20 seconds, or rotations per movement, depending on the requirement. To regain my splits, I am using x-ray paper and resting on the right, middle and left for about 2 minutes each. Eventually I'll go up to five minutes, but so far it is still excruciating.
The most important part of stretching is know your limit. If you tear something stretching, you have gone too far. So be careful when first exploring what stretches are recommended here. You will find many stretches to actually be detrimental. For example, placing your leg behind your back with the other extended can dramatically raise the likelihood of tearing your ACL, so always be aware of the risks associated with certain stretches.
Know your body, and it's limits. If you have an injury, or a disorder in say bone structure, you can seriously risk permanent injury from using stretches which while beneficial to others, can be harmful to your own body.
And in that, choose training partners who will assist and push you, not force. Your body is resilient, but a lot of MA people have a gung-ho attitude where they take pride in puking after practice. Pushing oneself gives growth, stretching oneself only does harm, and in the long run retard. Have trained for nearly two decades, I find it a BLESSING I have no injuries which are long term, and inhibiting my martial arts which is a very, very common tragedy.
A lot of the people I associate with are martial artists, and there is a particular individual I have trained with for many years, who is suffering many debilitating problems to his body from subjecting it to improper training. While he may be a beast in a confrontation, we have a nickname for him of 'old man Alec'. Fortunately he has found stretching, focused on realligning joints over time, to be very beneficial, but he is doing this with the assistance of a yoga chiropractor. Alec btw is 19.