Qt website changes too, for a long time could not find a previous version of the download address, from the Baidu search download to the following address.
Download the Qt: http: //download.qt.io/archive/qt/
Also we found at this address you can download some of the offline version:
https://www.qt.io/offline-installers
It contains the current version, 5.12.x version, 5.9.x version, Qt Creater, Other downloads and Pre-releases
