It's popular because it creates an interesting dynamic. Snape has always had a complex relationship with Harry in the original series. When Harry is deaged, it gives Snape a chance to start anew and perhaps be a different kind of influence. It also allows for exploring Snape's more nurturing side which is a contrast to his usual cold persona.
There's 'Snape and the Deaged Savior'. In this fanfiction, Snape discovers deaged Harry in an unexpected place. He takes on the role of caretaker with his usual grumpy exterior but deep down shows a lot of care. He teaches Harry magic in a different way than the normal Hogwarts curriculum, which makes for an interesting read.
Snape's character may become more empathetic. At first, he might be his usual cold self, but as he takes care of Harry, he starts to understand Harry's situation better and shows more kindness.
A possible plot could be that Draco has to figure out how to reverse the de - aging spell on Harry. In the process, he might have to seek help from unexpected sources, like some old family magic or even some of the more reclusive wizards in the wizarding world. This journey could bring him and Harry closer together as they face various challenges and obstacles.
One possible plot could be that Harry discovers a time - traveling accident has brought baby Snape to his present. Harry, being the kind - hearted person he is, takes it upon himself to care for the baby. He has to figure out how to feed the baby, keep him clean and comfortable. Maybe he also has to protect baby Snape from any threats in the wizarding world that might target Snape even as a baby.
The plot might involve Tony taking on a paternal role towards deaged Loki. He could start by providing basic care like food and shelter. As time passes, he discovers that Loki has a fear of being alone in this new state. Tony then tries to help Loki regain his confidence. They might also face external threats as some villains might try to take advantage of the deaged and vulnerable Loki. So, Tony has to protect Loki while also trying to reverse the deaging process.
Mainly, it's Bucky Barnes and Tony Stark. Bucky is the caretaker here. He has his own history, including his time as the Winter Soldier. Tony, on the other hand, is the deaged character. He's known for his intelligence and wealth in his normal state, but now as a deaged person, he has new vulnerabilities. While other characters might show up in the fanfiction, these two are the core around which the story is built.
He could also develop a better understanding of the 'light side'. Taking care of Harry, who represents so much of what the 'light side' stands for, even in his deaged state, makes Draco question his own alignment with the 'dark side'. This internal struggle can lead to significant character growth.
Well, at first Harry might be very suspicious. But as Snape shows consistent care, Harry could start to see him in a new light. Maybe he realizes that Snape has more layers than he thought. He might begin to understand that Snape's past actions were more complex than just being mean to him.
There might be a fanfiction where Harry takes care of Snape after a battle. He could be the one to tend to Snape's wounds, both physical and emotional. Since Snape has always been a rather closed - off character, Harry's attempts to get through to him and provide care could be a great source of character development for both of them. Harry might use his Gryffindor courage to break down Snape's walls.