All Qt versions up to 6.4.2 installed the release versions only of all needed plugins for my app when compiled in RelWithDebInfo mode.

windeployqt 6.4.3 from the Installer (msvc2019) suddenly installs *d.dll plugin versions (in addition to the normal, release ones), but only for the tls directory.

6.4.2 has:

qcertonlybackend.dll
qopensslbackend.dll
qschannelbackend.dll 

6.4.3 has:

qcertonlybackend.dll
qcertonlybackendd.dll
qopensslbackend.dll
qopensslbackendd.dll
qschannelbackend.dll
qschannelbackendd.dll

Maybe the double 'd' from backen'd' is throwing something for a loop?