At the moment, ToC updates are completely lazy: You create, or update, your ToC, and then LO forgets about it until you next manually request an update. There is no indication of when the ToC goes out-of-sync with the rest of the document.

I suggest that LO supports tracking this situation.

Specifically, that could be used for a visual indication on a ToC of its being out-of-sync, so you know you need to update it. It could also be used by other code which may want this information for its decisions, e.g. the suggestion in the discussion on bug 44448, to warn about an out-of-sync ToC before printing.