I don't think Google Sheets is a good example. The new version removes a separator but adds a bold title to the group of inputs related to formatting options and that is in itself a type of separator. The separator delineating the "Add another rule" touch area at the bottom is also removed, but if the touch target click target is expands beyond the text area of the new button, then that's fine. The user shouldn't have to be concerned whether the user has fat fingers anyway.
As for other places where you see the removal of separators, it really depends on how's the design.
- First of all, remember that perceived usability is often dependent on appeal. A clean but bad design that is well received in terms of aesthetics can be perceived as more usable than another better design with a low aesthetics factor.
- Second, as I indicated in my note about the Google Sheets example, you can separate information areas in other ways other than the use of plain visual separation lines
- Also note that separating lines can be overused, or lines can have excessive contrast, thus cluttering the design
That said there's instances where there may be a deficit in the visual division of information in a layout. One such case (IMO) is the Agenda app:
How many notes do you see in this screenshot?
There are some elements that serve as visual separators (the yellow dot and the date) but these are too weak to aid in quickly distinguishing the different notes. Titles don't count, because a single note can have multiple titles with a font size and weight similar to the main note title. Also note the strong visual competition from tags that further diminish the separating effect from said elements.
So yes, there's instances where killing visual separators can go too far, but abusing visual separators is also not good. Testing is key to determine whether there's really an issue or not in specific cases.