5.15, 6.2.0, 6.3.0, 6.4.0, 6.5
Qt documentation assume, that if a class isn't marked explicitly as thread-safe / reentrant the user can't assume it is. However, in fact Qt docs for many classes don't mark them as e.g. reentrant, while they are reentrant in fact. Examples of classes that miss the reentrancy marker:
QTimer (only selected static methods are marked reentrant, but the whole class seems reentrant too)
QPlainTextDocumentLayout (as his direct superclass QAbstractTextDocumentLayout is marked as reentrant)
Most probably there are more.