if it isn't thematically related to the current story, it doesn't do much for you
I think so. Maybe? If you and I are both focusing on the "thematic" part, here.
All stories have plots, even SoL stories. Those plots could be walking to school, attending a lesson, then going home on repeat, but thats still the plot.
Lets say I'm reading a SoL highschool romcom. I'm reading it for the comedy, and the fluff, most likely. Suddenly, on ch 21 of the story, strange forshadowing occurs, and by the end of ch 22, aliens invade. The next five chapters are about how everyone awakens superpowers to fight said aliens. It doesn't matter how amazingly written the aliens are, or the combat is. It would off-put me, because I wasn't there for heart-pumping action to begin with.
Like, its another matter if its a Highschool-SoL romcom where aliens are introduced from ch1 and the synopsis, because then I'm expecting them and it becomes a fundamental part of the story. I'm expecting aliens and I'm reading the story partially because of them.
I think its the same for tragic backstories. Halfway through a story, I'm interrupted by a completely different story which has different setting, plot, and themes running through it compared to the original. It doesn't matter how good it is. It puts me off because it wasn't what I started reading it for.
Again, lets take another, less extreme (non-existant) example.
Lets say we have some dramatic love story between some princess and her knight in shining armour. Why am I reading it? Probably for the interaction between the two main characters. What if, at ch 30, the author goes on a 5 chapter tangent on how the knight was ill-treated by his stepmother or something ten years ago? What does it have to with the interaction between him and the princess, except for the princess to say two or three lines of consolation to him at the end of it? I didn't pick up said story to read about a knight being ill-treated by his mother, so it doesn't interest me.
Now, lets say princess gets kidnapped along with the knight, and locked in a room. Then, the knight lock-picks the door, and as an offhand comment, mentions that he learned it ten years ago when his mother used to lock him up and starve him. Thats fine, because its relevant to the plot. we don't need the five chapters of detail to explain that his mother used to mistreat him, and any relevant information can just be brought up when needed. That way, the focus of the story never shifts off the relationship between the princess and her knight, so it wouldn't feel off-putting to me.
Alternatively, said tragic backstory could be mentioned in side-chapters or so, because side-chapters are optional and I could ignore them If they don't suit my fancy.
