They're the same as the Qt5 dictionaries. To get the ones Paul pointed to means doing a git clone. Easier to get the ones in @unixroot\usr\share\myspell. Either way they need to be converted with qwebengine_convert_dict.exe, either the one in @unixroot\usr\lib\qt5\bin or x:\qt6-6.2.x\bin. See my previous post.
I'd post the dictionaries directory, but zipped it is 35MB's. If you need an individual one, ask.
Actually here's my dictionaries, various en, now removed
Edit: figured out how to automatically create the dictionaries during build, so here's the ones I think we would like to have.