Hi,
This is essentially the same bug as #958747, but now with Qt 6 instead of Qt 5. Basically, it appears libqt6texttospeech6 is not useful without a speech plugin installed, some applications using this library don't report this error to the user effectively, and no package relationships would cause a speech plugin to be pulled in (not even as a transitive Recommends or Suggests).

In Okular for example, on a conventional Debian system, one may go to Tools→Speak Whole Document, but Okular will do nothing and fail silently without any error indication. Only if one opens a terminal they may find the following message on the console:

The commit https://salsa.debian.org/qt-kde-team/qt6/qt6-speech/-/commit/70d365d2113732678762ec0bc539ffaadd69fb17 basically fixed this, but only for qml6-module-qttexttospeech. Okular doesn't need that package, just libqt6texttospeech6, and indeed, there's no package on my system that wants the former (as determined by 'aptitude why').

It appears this problem has been partly papered over, because a few packages have been explicitly adding their own relationships on a speech plugin when they shouldn't need to. qt6-speech-examples directly depends on at least one speech plugin, and Calibre (the e-book program) has directly added Recommends on a speech plugin.
I would appreciate if this could be addressed in Trixie; this problem is hard for a user to identify. I only recognized what was happening because I was the submitter of #958747 six years ago (wow, time sure does fly...)