I think I found a bug with MSSymbolMaster isSafeToDelete, it's not detecting symbol overrides within nested nested symbols. Below is the report I submitted, but I am sharing here to make sure I am not missing something, and for visibility. Example file is available in the Dropbox link under Notes.
MSSymbolMaster isSafeToDelete is not detecting symbol overrides within nested nested symbols
Steps to Reproduce:
Create a symbol (like a list item, call it "Symbol B") which contains a nested symbol (like an icon placeholder, call it "Symbol A"). Place Symbol B in a new symbol (like a list item group, call it "Symbol C"). In Symbol C, override a Symbol A instance with another symbol (like an actual icon, call it "Symbol D").
Navigate to Symbols page, select Symbol D (which is only used as an override of a nested nested symbol), and tap Delete.
Sketch prompt indicating symbol is in use, and confirming deletion.
Symbol is deleted, displaying the red lines in the override indicating the symbol is missing.
In this linked example (https://www.dropbox.com/s/6shtevht11f375z/isSafeToDelete%20Bug.sketch?dl=0), navigate to the Symbols page and delete the Icon/Favorite symbol. Note the lack of prompt, and the red lines on the artboard on Page 1.