A sad violin song in most of flashback scenes like Obito's flashback and the storybook from Jiraiya There's a violin track in Naruto Shippuden that's kind of sad or emotional. It starts with a little guitar or banjo, then continued with a violin.

Up until season 18, most of the time the song comes up in a flashback, like Obito's flashback, and in the storybook from Jiraiya, and even more of them. One example is in episode 440 in the book from Jiraiya, the song starts at 17:17. Another example is from episode 464, around minute 17.

I have checked all the OST, but I couldn't find it.

What is the name of the song?
This was not released in any of the official OSTs. At the very least, it doesn't appear in any of the tracks in this list (as of 2018).

There are some unofficial versions online, such as this one, titled "Minato saves Kushina".

I remember this song as that nostalgic-sounding track that is mostly happy, but has just a touch of bittersweetness, and also that really good violin bit.

Not to be confused with "Let Me Tell You a Story from My Childhood", which sounds similar and also has some very nice violin parts, but unlike the first song, is more lighthearted and cheerful.