Concretely in the case that I attached here (it's a real case), the chapter ends on page 39. Page 38 starts a new subsection that overflows into page 39 but would have also fit into page 39 as a whole.
The question is: Should I prepend a \newpage? Or should I trust in the wisdom of LaTeX's auto-formatting?

\newpageat the wrong location would look terrible. If it is only a subsubsection and there are multiple lines and not just one it should be ok. For oneliners there are penalties, see How do I prevent widow/orphan lines?. – dexteritas Nov 03 '22 at 09:20