When the object (`lv_obj_t` ) is deleted by `lv_obj_del` function, are all of this object’s style(lv_style_t) automatically deleted?

When the object (lv_obj_t ) is deleted by lv_obj_del function,
are all of this object’s style(lv_style_t) automatically deleted ?
do i need to use the function lv_style_reset

No; lv_style_t objects have their own lifetime since they can be used on many objects. You will need to use lv_style_reset on them.

What about styles added to objects? As far as I understand every time when I call lv_obj_add_style() for this style allocated memory which is assigned to object-style_list [_lv_style_list_add_style() - lvgl_7]. Isn’t the obj->style_list removed/freed when the object is deleted?

The style list holds pointers to the styles themselves. The list is freed (since it’s part of the object) but the styles still need to be freed manually.